1985 Eldorado Engine Stumble & Surge

[Elwood]

I recently purchased an 85' Eldo w/ 84,000 miles, HT4100 gas engine. After a month of effort can't cure my last drivability problem. Symptom is an off-idle stumble (most noticed from a standing start) and low speed cruise surging (25-40mph). I have replaced: cap, rotor, ign. modual, plug wires, plugs, dist. cap harness, throttle possition sensor and harness plug, catlytic converter, exhaust resonator, and various and assundry other minor items. Most recently I have removed and cleaned the EGR twice. This effort has eliminated several other issues but still have the stumble and surge, only after the engine is at full operating temperature (i.e. >12-15 minutes drive time). Also observe erratic "Fuel Info" display with "instant" mode selected, display will rapidly indicate low single to high double didgit numbers and occasionally display various alphanumeric combinations along with intermittent lighting of the "Range" mode light or going blank for up to 1 second, also only at temperature and only when surge is present.
The car averages 18-19mpg on my commute and >22mpg average highway.
Thoughts anyone?

[Elwood]

Oh, and I forgot to add, after a 45 minute mostly highway drive on our first 80+ degree day and 10 minute shut down I started the car and put the trans in drive but the car would not move. I then noticed that in the left side "information center" the top left and bottom right red lights were lit. I did not have my reading glasses and couldn't read the errors so I shut the car off and left it over night ( I was home and wanted to move it but didn't need too). The next day everything worked as before and have not had this problem reoccur, but haven't tried to start and drive it hot.

[steelybill]

One thing I would add to what you have already done, is check the fuel pump pressure. There is a capped-off port in the fuel line, not far from the TBI where you can plumb in a gauge. The book sez the pressure should be between 9 and 12 lbs at that test point, in all speed ranges.
Some mention is made of the port in the manifold that the EGR valve is mounted on, for a build-up of "stuff" in that port.
Also hoses to the MAP sensor and really, all vacuum hoses for visual condition.

The spray pattern of the injectors can also have an effect.. In a throttle body, with the air cleaner off, the spray pattern from the injectors can usually be seen to some degree

Hope this helps.......

BILL

[Dave S Texas]

You posted nothing about trouble codes being present in diagnostics.

[steelybill]

Like Dave mentioned, you posted no trouble codes.. If you can get some codes it will be easier to help you. Do you know how to enter diagnostics??
with the ignition on, engine off, press "off" and "warmer" buttons both at the same time, momentarily. First is the seganment check, then status will begin with codes "E", and after it runs thru them, "F" codes , if any, will come up. When those are through, .7.0 will show it;s ready to go farther into checking the system.

The system in these cars can check just about anything when the right sequences are used. I know only the simple things that I had to learn about it to fix mine.

BILL

[carnut]

You say you've replaced the throttle position sensor. I presume you adjusted it to spec?? Thats the varible resistor that determines fuel delivery and how much. I agree with having you get the fuel pressure reading first. If at spec, I suspect TPS is out of range.

[steelybill]

The book says to make "sure the TPS pick-up lever is located ABOVE the tang on the throttle actuator lever". This info in the book suggests that maybe it's possible to get it wrong. Just thought it was worth mentioning that.

Vacuum leaks are also mentioned. Hoses from 1985 could be deteriorated. They were on mine

[Elwood]

Thanks for the input guys. The TPS replacement was done due to a previous group of problems including a TPS fail error code (I don't have my book here right now so don't have the code number). The tang is possitioned correctly and the sensor was zeroed at 0.65 vdc (mid range of the requirement). Since my last attempt, cleaning the EGR twice, I have logged no error codes but the loss of power when I reset the ECM. I'll see if I can find a fuel pressure gauge and check that this weekend.
Does anyone have an instrument cluster out of a car who could look and see what the upper left and lower right indicators are on the left side "Information Center"?

[Dave S Texas]

I have logged no error codes but the loss of power when I reset the ECM.

If you 'reset' the ecm by disconnecting the battery....bad idea. Doing that: erases the RAM(learned library) and reverts the system to the PROM. That ROM programming was used for a brand-new vehicle/engine/sensors. Trying to get a 29yr old engine system to run on the values of a brand-new engine/sensors isn't the way to go. Put miles on the engine and let the RAM rebuild.
May I suggest you buy a Cadillac Factory Service Manual(not a haynes/chilton) for your car? ebay always has a slew of them...$25 and up. Having one to use is 'handy'

[Elwood]

Thank you Dave, I just bought a book on e-bay so I'll have some reading to do next week. I did reset the ECM by lifting the negative this time. I haven't had the will to drop the panels under the dash and look for the fuse, and I've had to disconnect power a couple other times since I got the car anyway so I'm hoping the drive time I do have on it (~1000 miles over all) since I started fixing things is enough to restore the RAM. It probably has 200 on it since the last reset and still have the drivability issue at full operating temperature.

[steelybill]

I dropped both under-dash panels off until I was done fooling with the problems. It's also a good time to get familiar with what's under there while it's accessable, for the relay center etc.. Your manual has a parts locator page, and a fuse block page etc.

[Elwood]

Okay, I bought the 1985 final edition Cadillac service manual and have read it cover to cover, run all the diagnostics and still have the stumble and surge drivability issue with no failures detected through diagnostics other than one occurrence of a soft error #23 which seems to have registered during my normal commute. The “mystery “ information center indicators I documented earlier were due to the engine dying immediately after hot start, which has now occurred 3 or 4 times (had the radio too loud and miss-diagnosed the first instance). The surge condition seems more engine RPM than throttle position related, i.e. if cruising at 30-40MPH and experiencing “significant” surge/pulse/hunt conditions along with extremely erratic “fuel information” data a change to “drive” (3rd gear) from “overdrive” with no adjustment in throttle position immediately cures the symptoms. Similar response is observed at 15-25MPH cruise in “drive” when the transmission is shifted to L2 (2nd gear). The car still runs great in all other conditions and is still getting “typical” fuel economy (~19 city, ~23 highway). One other significant observation is that upon release of throttle to coast down from nearly any cruise speed, and even if above conditions are occurring, the fuel information display stabilizes at “reasonable values” for 5-8 seconds and then reverts to rapidly changing blank/numeric/alphanumeric values and random lighting of the “Range” mode indicator. The book doesn’t seem to have a cure for these symptoms so I’m all ears.

[carnut]

I think the biggest clue is the blanking out of the displays. I would suspect either the climate control head or the mileage display head as faulty. I.E. loosing the ground signal. On this early computer system, I've never found it necc nor have I seen an issue with it relearning parameters after an ECM disconnect, unlike my later 91 Allante which requires a structured idle learn procedure. I think it would be a good idea to drive this vehicle with the fuel pressure gauge positioned to be able to confirm steady pressure when this surge, stumble occurs. One other thing, check and adjust the TV cable at the throttle body to the trans. Early upshifts cause cause a surge to be felt. You can monitor torque converter lock up thru the orange dots in the climate display as driven as well. Is it locking up too early??

[steelybill]

If in fact one of the display heads is faulty you may not be able to get the info you need. I've seen those on ebay, but luckily didn't need one (yet). Mine goes nuts occasionally. I just tap the buttons a bit and the display seems OK again. Must be the switches on the PC board in those that act up.

[Elwood]

Thanks again for the feedback. I didn't mention that I was able to get a fuel pressure gage on the car a couple weeks ago and pressures were well within the acceptable range at all RPM's, however I didn't try piping the gage into the cabin for a drive test. Also confirmed that the injector spray pattern was correct. Given the symptoms I think I'm still chasing something other than the fuel system. I also believe the display heads are probably functioning correctly since I was able to run and properly read all the diagnostics listed in the service manual and erratic displays only occur in conjunction with the erratic engine function. The time to onset and severity of this condition has increased with our increasing temperatures, and the dying upon hot start is routine now so my guess is that something is going to start setting error codes soon. The new "die on hot start" is odd to me also. The car starts easily but dies as soon as the key is released to the run position when hot. I can then immediately restart the car and it will run fine, accept for the stumble and surge. I noted in another thread a suggestion on testing for EGR tube blockage by manually stroking the EGR valve stem at idle and confirm that idle quality diminishes. Worked perfect. I could easily kill the engine with a small opening of the EGR.
The transmission functions perfectly with converter lock occuring properly for both throttle possition and speed. As noted, this surge condition is displayed at lower speeds in 3rd gear which should disable converter lock.