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Prom needed/High speed great, mid to idle speeds terrible.

4K views 36 replies 8 participants last post by  gsim 
#1 ·
I have an '84 Sedan DeVille, 4.1 liter engine. Trouble on hwy 500 miles from home. Drove it home. Replaced bad MAT sensor. Was making car run lean, rough below 70 mph, would not idle. Replaced O2 sensor while I was at it because was original. Got new trouble code 52. So replaced ECM (Engine Control Module) Still runs rough at lower rpm and will not idle and still have code 52. Same exact characteristics as I had while out on the road when it first showed up. I am afraid that the P R O M chip is bad and Cadillac no longer sells that. Anyone know where I can get one? Or does anyone have an idea if 52 is a bad ECM or bad P R O M? The 'Service engine soon' and the 'Service engine now' both light up when I read codes on the climate control panel. I think that is pretty much telling me that the ECM is still bad, which leaves only the P R O M since I have a new ECM in now.

Gerry
:crying2: (just replaced exhaust, cat convertor, tires, shocks last year! Only has 124,000 on the clock.)
 
#3 ·
Re: Prom needed

Thanks, drmenard. My part # for the ECM is 1226028. The one I put on is a reman. There were no visible numbers on my P R O M, unless it was on the obverse side which is pressed into that plastic frame. Sigh. I had high hopes of getting her running right again. No rust, rarely spent a night outside, chrome great. Seats good and all power stuff works and never have even touched the air or added freon. Never have seen a Deville of that era in a junkyard in the 12 yrs I have had this one. Only saw Fleetwood Sedans, that era and only one of those some 60 miles away from here.
 
#5 ·
Re: Prom needed

Thanks, drmenard. My part # for the ECM is 1226028. The one I put on is a reman. There were no visible numbers on my P R O M, unless it was on the obverse side which is pressed into that plastic frame. Sigh. I had high hopes of getting her running right again. No rust, rarely spent a night outside, chrome great. Seats good and all power stuff works and never have even touched the air or added freon. Never have seen a Deville of that era in a junkyard in the 12 yrs I have had this one. Only saw Fleetwood Sedans, that era and only one of those some 60 miles away from here.
I have nothing to offer other than to say don't get discouraged, these guys here will get you through this. I am always amazed at the knowledge available here. That sounds like a car worth the wait!
 
#4 ·
Re: Prom needed

From what I remember about code 52 is that it is telling you that the battery was recentlly disconnected, don't think it means the ecu is bad.

I think if you have a bad prom or ecu the car would not fire up...? (I'll let the more quilified people verify that).

Also when you enter diagonstic mode on these cars the service now & service soon lights do come on.

I think you may have a different problem I would say either fuel system or electrical system.

Good luck with your project.
 
#6 ·
Re: Prom needed

"trouble 500 miles from home" we need a better description. Code 52 is an old (soft) code. Means that the battery was disconnected somewhere in the cars past. Maybe when you swapped out the ECM? When you pull codes, the codes that display 3 times are current ( hard ) fault codes, if they only display twice, they are history (soft ) and should be erased. Do you have a FACTORY service manual? When you transferred the PROM, did you possibly bend a pin? as you inserted it. easy to do! The ECM itself fits many GM cars of different years, the PROM is specific to your year and car type. I doubt you needed the ECM but now is too late. Did you have a hard (3 displays) code for the MAT sensor prior to replacement? What hard codes do you have now?
 
#7 ·
Re: Prom needed

"Thanks AElayyat and thanks CarNut. Will poke around some more. Today I will spray cleaner into throttle body throat as I run the car. I will also pull the codes again and this time I will count the number of times that they display. OH, I did run a can of fuel injection cleaner thru the thing when it first showed up, hoping that it was maybe a dirty injector. No help, but it was a hail mary being that we were 500 miles from home.

The 'Trouble 500 miles from home' was the same as now. Once warmed up, would not idle. Once warmed up, would not re-start without starter fluid. Ran as if it was starving for fuel at lower speeds and all the way to 60 MPH. At 70 MPH it was as smooth as could be. At that speed I could not tell that I had a trouble. To me that eliminated leaky line/pinhole, etc or fuel pump and fuel filter.

All that I have for reference is a Haynes manual. It shows a code 52 as either ECM box or P R O M chip bad for all models including v6, HOWEVER it did NOT include the Deville. I could not find a number assigned for a Deville for ECM failure in my book. I finally took that to be a typo. Reason being is that it also said that if 'service engine soon' and 'service engine now' lights came on while I was getting codes, that indicated either ECM or P R O M failure.

I think that disconnecting battery wipes stored codes clean or at least according to my Haynes book it does. I will run it and check codes this AM. The codes do not come on unless the car is being cranked or is running. Just turning on the key and pressing the 'warmer' and 'off' buttons will NOT bring up codes. The car has to be running or cranking. I recall that when running a fuel delivery trouble 3 yrs ago. Could not get codes to come up until I cranked it. ( On that one, it was the fuel hose knocked off of the fuel pump by an attempt to siphon gas from it some 3 weeks earlier.)

Gerry
 
#8 ·
Re: Prom needed

Every 84 Cad I've owned will display codes, key on, engine off. Are both injectors spraying when its running? That Haynes manual needs filed in the trash. I would verify that the MAP (manifold absolute pressure) hose is intact as it goes from the connection on the throttle body to the MAP located by the ECM underdash. That hose clips onto a long fender brace, passenger side underhood, and travels thru the dash to the sensor itself. Without MAP reference the car can run poorly. Verify vacuum is present at the throttle body connection, they can plug up with carbon. Spraying cleaner thru the throttle body is a waste of time. The spray of fuel above the throttle body itself cleans the throttle body bore and throttle blades as the car runs.
 
#9 ·
Re: Prom needed

Thank you, Carnut.

My car has never, ever displayed even one code unless either running or cranking. I have had it for 12 years. I will look into the hoses anyway tho. I did tighten the alternator belt while we were out of town so I will look real good around that area to see if I did something to it. Did not have any trouble that day tho. And my wife says that this trouble came in on her once BEFORE we ever left town. It ran perfectly for 4 days while we were out of town tho. When I replaced the ECM, I did see two vacuum things behind the glove box liner adjacent to the ECM itself. One had a vacuum hose running to it from the area of the heater core/evap core region. Other side had no hose on it at all, but I carefully looked around with a flashlight and felt around in that area as well and never could find a hose that was disconnected. I have done no work in that area, so I have no reason to think that it suddenly fell off and disappeared. I will look again though just to be sure. Both of these vacuum devices are switches with about 2 or 3 wires on each and both are mounted on a small bracket back-to-back. As I said, one has a hose that is intact and one does not have a hose on it at all, but has 2 or 3 wires going somewhere. I thought that it was maybe for an option that my car did not have. Regards Haynes manual I could not agree more. I tried to get factory manual at our library but all three are overdue for a long time now, and the librarian thinks that they will never be returned. I will look for one on eBay.

Kindest regards and thanks again,

Gerry
 
#10 ·
Re: Prom needed

You are describing the MAP and BARO sensors behind the ECM. The alternator belt usually becomes loose once the smog pump belt is loose. Notice that the alt belt only runs around the water pump pulley. If the smog pump belt is loose, glazed, or worn out, it will cause the alt belt to squeal or slip causing low voltage at any RPM above idle. Tightening just the alt belt will not correct the slippage. Best practice is to loosen the alternator then tighten the smog belt, then retighten the alt. belt.
 
#11 ·
Re: Prom needed

Thank you, carnut. All belts are new as I did replace the water pump last winter. There was no hint of any trouble after that. Have just re-checked once more and all belts are properly tightened. Alt belt on my car is strictly run from the water pump hub as you said and yes the smog pump drives the water pump via its belt. The MAP hose does go from where you said directly to the MAP sensor next to the the ECM and those wires are firmly attached to the plug and the plug is on tight. The other one that you said is the BARO sensor looks as if it has never had a hose on it. Maybe it reads barometric pressure through that neck/nipple that looks like it was made for a hose? There is no stray unconnected hose there. I am wondering if maybe my low speed jet is just in trouble? There are only two jets so I figured that one was for cruise-high speed and one was for cruise-low- idle speeds. When the rpm is up, both have a steady stream of fuel vapor and when it stumbles, both are interrupted more, but it seems that the one on the passenger side is more sporadic than the one on the driver side. Also it will periodically run up to a higher rpm and then stumble back down, all without any input at all. This is while it is sitting and supposed to be idling. I had to back out the throttle lever stop near the idle control motor in order for it to idle at all without someone in it keeping it going with the foot feed. So if it was running right, it would be idling way too fast now. Again, the car runs super smooth at 70 mph and above and begins to get rough and surge and wane around 60 and below. Worst is low speed while going uphill. If car is warmed up, it cannot be re-started without starter fluid!
 
#12 ·
Re: Prom needed

Those are not jets, those are injectors. They should spray equally at all times. I recommend a fuel pressure test. Again, what codes are being displayed? If the belts are tight, you should NOT be able to rotate the alternator pulley by hand against normal rotation (engine off) If you can rotate the alt. pulley at all then the smog pump belt is loose still.
 
#13 ·
Re: Prom needed

Thank you, car nut. Old timer's terminology. I am aware that they are injectors but still I will call them jets. They go sporadic when the engine stumbles down and gets to running rough. That could be electronic trouble too since that is what governs their volume, etc. It runs very well at 70 MPH and above. I figured that would eliminate fuel pump, fuel line leakage, and fuel filter. It will run for hours at 70 or more without any indication of trouble. From 60 mph down it progressively gets worse and going up a grade, that is amplified.

I thought that maybe a fuel pressure test would be good, but that even better might be a voltage test across the leads on the two injectors? I can put an analog voltmeter across them, one at a time and observe the voltage as the thing runs. I set the idle up to the point where it will at least stumble along and not quite die. Every so often it surges and runs RPM up right after nearly dying, all by itself with no input. There might be a volume measuring sensor in the intake somewhere that is on the fritz.

Regarding codes, I still get the exact same reading, and I only get it with the thing running. It is as follows:

1.8.8 52 1.8.8 52 7.0 If I keep pressing the two buttons down it will read 7.0 repeatedly after that. I suspect it it telling me that it has displayed all of the info it has. And as soon as the very first code comes up (1.8.8) both the 'SERVICE ENGINE SOON' and the 'SERVICE ENGINE NOW' lights come on. The Haynes manual says that is a sign of either ECM or P R O M failure. What can I say? I can find no other literature hereabout to refute that, and the local Cadillac dealership guys were sympathetic, but could not help me. They told me that after Cadillac became 'Government Motors' they sold all of their paper manuals on older cars to one person who came and loaded up all of the books before they relocated to their present place.

Belts are properly tight and the Alt cannot be turned whatsoever, and really could not even before I tweaked it while in Indiana.

So I will proceed with the voltage test this AM on both injectors and look to see if it wavers/dips down or spikes when it stumbles and tries to die. Fuel pressure will be a problem being as I will need to match the fuel pressure test port with something threaded. Wish it would take a spark plug thread tester as I have two of those.

Thanks again,

Gerry
 
#14 ·
Re: Prom needed

If you push OFF and HI at the same time, and hold them depressed, once in diagnostics, it will eventually display 0.0 meaning all codes are erased. Code 52 is a soft (history) code. Erase it. 18.8 means all digit segments can display, 7.0 means you are at the end of troubles codes display. With the air cleaner off and the adapter under it off the throttle body, look very closely at the 2 injector harnesses as they pass under the adapter. I've seen where they get pinched or partially cut there. That could account for the sporadic spraying. If I recall correctly the injectors operate at 5 volts or less, so a digital meter would be best. Both Service lights will and should display while in diagnostics, this is a designed in "bulb" test to make sure both will illuminate. Please hold off further checks until you get a factory service manual!!! There were at least 5 on E Bay yesterday. There cannot be any logic to your diagnosis without this book.
 
#15 ·
Re: Prom needed

Thanks, Carnut. Today I took an analog VM and read across both injectors. I was a steady 2 volts DC while it was running. Although the engine alternately ran fast and rough stumbly slow, the voltage remained the same. Once it died, the voltage on the Driver's side injector died too. However on the passenger side injector, the voltage stayed the same after the engine died. It may be that is the low speed one, if there is such a thing? Made me think that I have no ECM/PROM trouble.

Regards codes, I have never tried to hold down OFF and HI. My book said OFF and Warmer was the way to do it and I did it that way. I will try to look at it the way you say to see if it will read codes at all.

Regards the manual, I would love to buy one. Just yesterday I was on eBay and typed in 1984 cadillac auto repair manual. I was taken to a page with lots of old caddy parts and several Haynes books. At that time, I never saw even one factory repair manual. I will try again and will type in cadillac factory shop manual and see if I have as good luck as you did.

Thank you, Carnut.

Kindest regards,

Gerry
 
#18 ·
Re: Prom needed

Carnut,

I will have to read fuel pressure via a Schrade valve type fitting. Will have to buy a gauge for that. Looks like an old freon charging valve.

Seems odd to me that fuel pressure would fail at idle to low-medium , yet would not fail for hundreds of miles at 70 MPH or above. Can that even happen? Almost more like a faulty pressure bypass valve. At the higher speeds all of the fuel goes into the engine and none goes back to the tank. Any possibilities there?

Kindest regards,

Gerry
 
#21 ·
Re: Prom needed

Again there is no low speed or hi speed injector. The left one feeds the right bank, the right one feeds the left bank. Re read my explanation on holding off and hi. Holding those IN DIAGNOSTICS will erase codes after a 0.0 is displayed. Its not used to enter diagnostics but to erase old soft codes. Loosing voltage is a good clue. Did you check for bare wiring under the adapter? I agree, there should be voltage at both injectors. if the car stalls while running and voltage quits as well there is a problem in the harness to that injector.
 
#22 ·
Re: Prom needed

I did check the wiring and there are no bared wires shorts, etc. The voltage stays steady even while the thing is stuttering and stumbling down and when it is revving up a bit. The thing that was weird was that when the engine died, one had the voltage die too even tho the key was on. However the other had the voltage stay even tho the engine had died with the key on. That was what made me think that one might be for start/slow and the other for high speeds. However both stayed at a steady 2 v while the engine was running no matter the speed. The voltage did not vary even though the engine did vary a real lot in speed and smoothness. The fuel does sporadically stop when the thing is stumbling down even tho the voltage is steady. I can see that with a flashlight very easily. Made me wonder about the fuel pressure regulator.

I just cannot imagine the fuel pump or filter being bad if it will run flat out 70 mph and up to 85 mph with no problem at all. The slower you drive the worse it gets. I will check at an auto parts store the next time I am out to see if I can buy a gauge that will fit the schrader valve in my car. I want to watch that gauge while that thing is stumbling and stuttering and also when it is smooth and picks up rpm's. Is the regulator in the center between the two injectors or is it mounted elsewhere? I have seen them mounted on the fuel rail in most cases, but seems I was looking at a Pic in the Haynes manual that showed it to be in the throttle body itself, feeding the two injectors. There should be a line from it returning to the fuel tank.
 
#23 ·
Re: Prom needed

I'd look around for a dirty connection in a weatherpak connector somewhere. The pins to the intake manifold temp sensor can get a little grunge and make things act wierd. The sensor itself is a known problem with 4.1 RWD cars.

Many short or on edge sensor readings will not always show up as a code.

The factory manual's trouble trees are excellent. Do not try to out think them. Been there done that.
 
#24 ·
Re: Prom needed

Thanks, Ape Man. I will pull those apart and take a look around the vicinity of the Intake manifold. I did run a test on the fuel line and I saw a steady 5 or 6 lbs the whole time it ran. It alternately ran at a fast idle to stumbling down and almost dying. When it stumbled down and ran rough, I could actually see the gas vapor coming and going off an on, even tho the voltage and the pressure were steady the whole time. I have a steady 2 VDC and a steady 5 or 6# fuel pressure and neither wavers. I am wondering about the fuel regulator adjacent to the injectors. It is mechanical tho, no wires. Does anyone know what the three wire gadget on the passenger side of the throttle body housing is? A sensor of some sort?

Gerry
 
#26 ·
Re: Prom needed

Thanks, Ape Man. I will pull those apart and take a look around the vicinity of the Intake manifold. I did run a test on the fuel line and I saw a steady 5 or 6 lbs the whole time it ran. It alternately ran at a fast idle to stumbling down and almost dying. When it stumbled down and ran rough, I could actually see the gas vapor coming and going off an on, even tho the voltage and the pressure were steady the whole time. I have a steady 2 VDC and a steady 5 or 6# fuel pressure and neither wavers. I am wondering about the fuel regulator adjacent to the injectors. It is mechanical tho, no wires. Does anyone know what the three wire gadget on the passenger side of the throttle body housing is? A sensor of some sort?

Gerry
That's the TPS throttle position sensor. It's the same thing as a 3 terminal potentiometer. You can check it with a VOM. One wire on the center terminal, one on an end terminal. Run the throttle through it's range slowly and see the reading change on the ohms scale.

Those get flaky usually just off idle position first. I doubt it is causing your problem.

Remember voltage readings taken with a modern meter may be bogus. Modern meters don't put any load on a circuit. You can read 12 volts through dirt but not have 12 volts available to light up a tiny light bulb.

You can look at the spray from your throttle body injectors with a timing light for a good stop action view.

The fuel pressure regulators built into GM throttle bodies of that era were very reliable and trouble free unless tampered with.

The injectors are controlled through the ECM which is looking at distributor pulses from the pickup and module. They have 12 volts hot on one side through IIRC 3 amp fuses and the ECM completes ground for a pulse.

A good factory service manual describes all this stuff. I'm doing it from memory. Distant memory. I have not touched an HT-4100 for a long time.

My experience is make sure all ignition components are in top condition before chasing DEFI problems. Your engine has the cap buried where it isn't easy to check.

Spark plug boots on these era cars can break the wire internally if not given a half twist before removing from the plug. That internal break can cause a mis-fire under conditions of higher required firing voltage like climbing a hill.

Always check the easy and cheap stuff first. Good luck.

Off to diagnose a high voltage power supply on my ham transmitter.........................................Wish me luck.
 
#25 ·
Re: Prom needed

Carnut,
I just did look again at the codes. It definitely has to be the 'OFF' button and the red 'Warmer' button pressed simultaneously. No other combo will even produce a code reading of any kind or even a starting number. I am seeing 1.8.8 52 1.8.8 52 1.8.8 7.0


And I did succeed in getting readings with the key in 'Run' position without starting the engine. IT just takes a bit longer to start. Also I have both the 'service engine soon' and the 'service engine now' lights on the whole time the codes are flashing. My book (Haynes) says that means ECM or P R O M are bad. (a new ECM did not help)

I have not found a DeVille manual, but I did find a electrical manual on the Deville. I bought a 1984 final repair manual for supposedly all caddy models, but it has the Fleetwood symbol on it, so I am thinking that there will be little there to help me. It was only around $10 or so. Maybe $7 for the wiring book, factory also.

Both fuel pressure and voltage are steady even tho fuel flow is not.

Gerry
 
#27 ·
Re: Prom needed

Thanks ape man. Thing is this car will run flat out smooth as silk at 70 MPH and above all day long. I drove it some 500 miles with this trouble in order to get home. it does not act like an ignition trouble, AND I can actually see the fuel being interrupted while both the voltage at the injectors is steady and the fuel pressure is steady too. So I am puzzled. I am going to check that TPS thing though. It could be that it is cruddy internally at the lower end position and is reading correctly at the higher speed end. Will be easy to test using an Ohm meter and it should be a steady constant and predictable slope to the readings. If dirty/burnt in one position but ok in the higher ranges, it could explain what is happening maybe.

Gerry
 
#28 ·
High speed great, mid to idle speeds terrible.

I have a 1984 DeVille sedan with the 4100 engine. It runs well at 70 mph and above, but seems to be sometimes starving for fuel at lower speeds. Will really not idle at all. Needs starter fluid to restart once has warmed up. Drove it this way 500 miles to make it home. Have repl O2 sensor, MAT sensor, and finallly, ECM due to codes read. Still have code 52. Have steady voltage on both injectors of 2 VDC and have steady fuel pressure of around 6# (Low) Am wondering if fuel pump needs to be replaced, but cannot understand why it will run for hundreds of miles at 70 MPH and up with a weak fuel pump??? Am also wondering about TPS potentiometer being dirty/burnt maybe internally at the lower speed positions and not at the higher speed position?

Gerry
 
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