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95 Deville
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Discussion Starter #1
Hi, I've searched the forums for any other posters who might be having my problem, but I'm not finding any matches. Although it does seem that the Caddy 4.9's favorite pastime is doing a Speedbuggy Impression rather than motoring along. That being said, this is the problem my Cadillac is having. It all started about 3 weeks ago. I was dropping a friend off at his house, while chatting in the driveway I let the car idle for about 15 minutes. When I went to leave any application of throttle triggered a horrible stumbling/sputtering/backfiring/dying ensemble. After a moment of this racket the car died. I put it in park, started it, revved it, seemed to spin freely, so I put it in drive and it started to act up again. I turned it off, waited a moment, restarted it, pulled out, when I got up to about 35 miles an hour it started again and lost power. I coasted into a gas station. Figuring I had got some bad gas, I poured two bottles of fuel line dryer in it and continued home without issue. I drove it for nearly 2 weeks without issue. The second time it happened it had also come after a bout of sustained idling. It was during this time I noticed the heater in the car was blowing intermittently. Now I know the Climate control requires vacuum to blow, so I started looking for a leak, couldn't find one. Second theory was the Engine Coolant Temp sensor had gone bad and was telling the car the engine was at -32 and it was loading it up with gas, which would also explain the lack of heat. During all of this, the car was throwing no codes, no check engine light. So I spent $10 and threw a temp sensor at it. Hasn't helped. Second thought was fuel pressure. Put a gauge on the fuel rail, it's 40psi initially, 35 PSI at idle. I let the car idle until it started acting up (Took like 30 minutes of idling before a blip of the throttle started the dying fit again). When the car is stumbling and dying, it's pushing 40 PSI of fuel pressure. I did happen to have a new fuel filter lying around, so I swapped it in. The old one was OEM, but it did nothing for the fuel pressure. Second thought was a Catalytic converter/exhaust obstruction. I put a vacuum gauge on it, spliced between the throttle plate and the EGR Solenoid. At idle the car draws about 19" of vacuum, if I bring the revs up, it'll go to about 25-26". I let it idle for about 30 minutes with the gauge on there. This time it didn't start acting up, but at no time did the vacuum drop. So I'm guessing the exhaust is exhausting and theres no back pressure burping up the wrong way. I checked the map sensor, and noticed two things; First, the clip was broken off, so someone had been monkeying with it before, and second one of the plugs looked blackened. I threw down $40 and replaced it. This didn't solve the problem, but it has changed the nature of it. Before once the car had started stumbling/chugging it would just do it until it died out. Now the engine recovers. I drove around in circles for like 30 minutes before it started acting up, but now if I let go of the gas the car will assume a normal idle rather than continue chugging. It'll rev unloaded, and I can even drive the car so long as I seriously light-step the throttle. If I apply throttle at a driving-miss-daisy pace the car will move without a problem. If I throttle it like normal traffic, it starts chugging. In a fit of rage I've, started replacing Vacuum lines all willy-nilly. (The metal line that goes down to a solenoid on the transmission was especially useless. With a new hose the car seems to shift much better... right until the car eats crap and dies) All the while, still no Service Engine Light. Before you ask, I have actually changed the distributor cap, wires and plugs all within the last 3000 miles. I am totally at a loss at this point and open to suggestions... or offers to buy at deep discount..:banghead:
 

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1991 Cadillac Sedan deVille
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591 Posts
Have you checked the ignition coil and ignition control module? I would check the coil next time it starts acting up, and the module - go to a parts store and tell them to run the test a few times (it might fail when hot).

Another thing, go to the TPS parameter in the OBD and with the car off move the throttle and look for erratic behavior. The display should change numbers smoothly and go up to about 80*, and there should be no skips.
 

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Master of the Dark Art of Diagnostics
2003 DHS - two-2002 DHS, 2003 SLS, 1995 Sedan DeVille, 1989 Coupe DeVille
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19,292 Posts
the problem sounds ignition-related to me -

the ignition module is cheap and easy to replace -
and while you are in there -
closely inspect the pick-up coil -
pay particular attention to the little wires at the base of it -

those wires constantly flex with spark advance -
they can break at the coil -
so when the spark advances -
the wire loses contact -
and the engine starts to die -

when the spark retards - the broken wire touches back together -
and the engine recovers - and continues to run -
 

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1991 Cadillac Sedan deVille
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591 Posts
those wires constantly flex with spark advance -
they can break at the coil -
so when the spark advances -
the wire loses contact -
and the engine starts to die -

when the spark retards - the broken wire touches back together -
and the engine recovers - and continues to run -
There is no mechanical advance in these distributors, so no way the wires are flexing. However, I also suggest the OP inspects those wires. They tend to become brittle with time given the harsh conditions they are subjected to.
 

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White Diamond '03 DHS (with DTS floor shift)
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Now I know the Climate control requires vacuum to blow
Wrong. The blower is electrical. No vacuum required. The mode (distribution) is vacuum operated.


Before you ask, I have actually changed the distributor cap, wires and plugs all within the last 3000 miles.
Rotor too?
 

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This sounds completely like a problem I had with one my previous cars. Some days it would run great and then sometimes it would miss like it was on it's death bed. After doing what you did and to no avail, I checked the rotor and there was a small hole on it that was caused by an arc. Replaced the $5 part and the car ran like it was new. Try it.
 

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1992 Fleetwood S&S Hearse, 1993 Buick Roadmaster
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Also sounds like my dodge truck when the ignition coil died, and god-knows-how-many cars my customers have come in with having the same problem, only to replace the ignition control module and the problem stop. So I agree with Basscat, and further, many local auto parts stores can check your ignition modlue for free on many cars if you take it off and bring it to their store. Just call first and make sure the machine supports testing your specific unit. And if you get a new guy or some young kid, have them run the test until the part gets hot, maybe 5-10 times. Might not make a difference, but then again, it might. Hot ign. modules fail most often in my experience.
 

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95 Deville
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Discussion Starter #8
UPDATE: Sorry about the delay, life happened and the car just sat in the driveway for a bit. It appears the problem was the engine PCM. I found one for $20 on ebay and slapped it in. So far, 50 hard miles with nary a hiccup. I thought I might have had a bad ground to the fuel injectors, but I asked an ASE and he said the computer was the ground for the injectors so that's how we came to that conclusion.
 

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95 Deville
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Discussion Starter #9
Update: car went 90 miles before the problem reared its ugly head. The new PCM still isn't throwing any codes.. I'm about to give up on this
 

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1991 Cadillac Sedan deVille
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Did you test the ICM, pickup coil, and ignition coil as suggested? Those are important components and shouldn't be overlooked.

Have you also inspected the behavior of the TPS like I suggested in my 1st post?

You should be diagnosing things and not just throwing parts at it and wishing it'd magically fix itself. Many people gave you very good suggestions, but it seems like you didn't make use of this information. It seems like you just slapped a new computer (and bunch of other parts before that) in there and hoped for the magical fix? Well, that's not how it works... do the proper diagnosis.
 

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95 Deville
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Discussion Starter #12
I tested the Hall Effect Sensor according to the instructions on the Autozone Repair guide website. It says the acceptable range is 500-1500 Ohms. Came back 6XX. The TPS sensor went to 4.8X volts. Just shy of the 5.0 Mitchell/Direct says is the range. Never mind the fact that the TPS should have thrown code if it went south, P055 IIRC. It hasn't. Not that I trust the computer as far as I can throw it. The computer doesn't even record the fact the engine stalls out (That would be P095). I don't post regularly as I've been working (Way too much) quite a bit as of late and the car hasn't been a priority. As for throwing parts at the car, the MAP Sensor is the only Powertrain code it's thrown and that was actually burned. The PCM seemed reasonable, as someone I was talking to mentioned it when I pointed out the injectors aren't firing (That's why the fuel pressure goes from 35 PSI to 40 when the stumbling starts). Out of morbid curiosity, I went to check out the computer and found that the PCM had been removed from the car prior (Namely it wasn't bolted in, half the under dash fasteners were missing, the plug locks were broken, etc). Seemed like a likely suspect. As for 'Not wanting to do any diagnostics'... Here is what it isn't

It is not Exhaust Related. EGR Valve seems to be operating properly, there is no code, there is no back pressure building in the intake, Vacuum remains constant.

It's not Vacuum related. 19-21 inches of vacuum at idle, 25-27 when rpm is increased. That being said I did find some questionable vacuum lines, replaced them as I had the hose lying around. I mentioned earlier that the blower fan doesn't work if there is avacuum leak and someone pointed out the blower fan motor is electric. This is true, but if there is a vacuum leak and the blend door doesn't respond to the HVAC Control, this trips a code (A040) and as long as this code is listed as current the blower fan will not engage (I discovered this first hand).

It is not the fuel pump, relay, fuel pressure regulator, fuel filter. Again, 35+ PSI at the fuel rail.

It is not spark related. Besides the basics being new, the Hall unit seems to pass mustard. Also, I do have an adorable little spark tester that I put on the wires and they are firing even during the stumbling.

That being said, the current theory that's getting bounced around is a bad ground somewhere getting hot and shorting out. As I mentioned in the first post, the blower fan started to blow intermittently as a harbinger. I was originally thinking it was the HVAC computer losing vacuum, but now I'm starting to think it's shorting out as well. As far as I can tell the PCM and HVAC Blower only cross their paths at the main fuse block under the hood next to the washer fluid res.

The location is Topeka, KS.
 

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1991 Cadillac Sedan deVille
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I tested the Hall Effect Sensor according to the instructions on the Autozone Repair guide website. It says the acceptable range is 500-1500 Ohms. Came back 6XX. The TPS sensor went to 4.8X volts. Just shy of the 5.0 Mitchell/Direct says is the range. Never mind the fact that the TPS should have thrown code if it went south, P055 IIRC. It hasn't. Not that I trust the computer as far as I can throw it. The computer doesn't even record the fact the engine stalls out (That would be P095). I don't post regularly as I've been working (Way too much) quite a bit as of late and the car hasn't been a priority. As for throwing parts at the car, the MAP Sensor is the only Powertrain code it's thrown and that was actually burned. The PCM seemed reasonable, as someone I was talking to mentioned it when I pointed out the injectors aren't firing (That's why the fuel pressure goes from 35 PSI to 40 when the stumbling starts). Out of morbid curiosity, I went to check out the computer and found that the PCM had been removed from the car prior (Namely it wasn't bolted in, half the under dash fasteners were missing, the plug locks were broken, etc). Seemed like a likely suspect. As for 'Not wanting to do any diagnostics'... Here is what it isn't

It is not Exhaust Related. EGR Valve seems to be operating properly, there is no code, there is no back pressure building in the intake, Vacuum remains constant.

It's not Vacuum related. 19-21 inches of vacuum at idle, 25-27 when rpm is increased. That being said I did find some questionable vacuum lines, replaced them as I had the hose lying around. I mentioned earlier that the blower fan doesn't work if there is avacuum leak and someone pointed out the blower fan motor is electric. This is true, but if there is a vacuum leak and the blend door doesn't respond to the HVAC Control, this trips a code (A040) and as long as this code is listed as current the blower fan will not engage (I discovered this first hand).

It is not the fuel pump, relay, fuel pressure regulator, fuel filter. Again, 35+ PSI at the fuel rail.

It is not spark related. Besides the basics being new, the Hall unit seems to pass mustard. Also, I do have an adorable little spark tester that I put on the wires and they are firing even during the stumbling.

That being said, the current theory that's getting bounced around is a bad ground somewhere getting hot and shorting out. As I mentioned in the first post, the blower fan started to blow intermittently as a harbinger. I was originally thinking it was the HVAC computer losing vacuum, but now I'm starting to think it's shorting out as well. As far as I can tell the PCM and HVAC Blower only cross their paths at the main fuse block under the hood next to the washer fluid res.

The location is Topeka, KS.

Wait, the specs you state of 500-1500 Ohms are for the pickup coil, not the hall-effect switch. 600 seems to be within specs, if that's what you tested.

I don't know how you tested the TPS... the TPS receives a 5 volt reference signal, so for the output to have been 4.8V it would mean WOT (Wide Open Throttle).

Regarding the injectors, how do you know some of them are non operational? Have you tested their resistance (13-16 Ohms), or listened to them with a screwdriver to your ear/stethoscope?

Here are some pages from my '91 FSM regarding TPS adjustment. Does the voltage fall within the specs?



 

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In my experience ignition problems usually lead to misfires and backfires. Since you have a lack of power with no backfiring I would lean toward lack of fuel. It must be something with the injectors. I have never had injector problems with any FI vehicle i ever had so I don't have any advice in that department.

The intermittent blower issue could be the BCM trying to join the fun. That may be another problem for down the road but won't affect the engine.
 

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95 Deville
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Discussion Starter #15
Regarding the injectors, how do you know some of them are non operational? Have you tested their resistance (13-16 Ohms), or listened to them with a screwdriver to your ear/stethoscope?
When there was a gauge on the fuel rail, pressure increased when the hesitation started. Turn the key, 40 pounds. Car running, 35 pounds.. when the stumbling started the pressure increased to 40 pounds. It would seem the logical explanation is the injectors aren't venting the pressure, hence they are closed. I haven't checked the injectors yet, as it's obviously all 8 of them ceasing and not just one. I've wondered if one might be shorting out and taking them all down, but I can't imagine that wouldn't trip a code. It would be something to look into on my next day off.
 

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After the new PCM install, the car ran ok for a short while, degraded after some miles in closed loop. The most logical place to start here would be to drive around while looking at the fuel trims in the OBD system (rich or lean, under which conditions). That will start you off on the right track, rather than blind "test this, test that, replace that, etc".
 

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1992 DeVille
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I remember reading in my FSM that if the pcm doesn't seat properly it can lead to all kinds of strange running /drivability issues
 

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1991 Cadillac Sedan deVille
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When there was a gauge on the fuel rail, pressure increased when the hesitation started. Turn the key, 40 pounds. Car running, 35 pounds.. when the stumbling started the pressure increased to 40 pounds. It would seem the logical explanation is the injectors aren't venting the pressure, hence they are closed. I haven't checked the injectors yet, as it's obviously all 8 of them ceasing and not just one. I've wondered if one might be shorting out and taking them all down, but I can't imagine that wouldn't trip a code. It would be something to look into on my next day off.
You could be right about one or more injectors shorting out when fully warmed up. That would also take out the injector driver(s) in the PCM as well. That could indicate why your car ran good for some time after you replaced the PCM, and then it started doing the same thing. If you have such injector problems they could've taken the injector driver(s) in the newly installed PCM as well.

I would definitely start with checking the resistance of all the injectors when cold, and then do it after the car has warmed up, because injectors are likely to short out when warmed up. Compare the values. Specs are 12-16 Ohms. After you record their resistance, start the car again and use a screwdriver to listen to each of the injectors (it helps if you remove the air cleaner from the throttle body so you can lean over the engine for the right bank). Make sure they're all ticking steadily.
 
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