Intermittent Stalling Issue - 1998 Cadillac Seville [SOLVED!]

I’ve read through the post looking for clues and am inclined to focus on the original symptoms;
it was stalling when “turning into a parking lot or going around a sharp entrance ramp” and the engine speed dropping and then rising again when coasting at 25 mph. I also noticed that being low on gas seems to have contributed to the problem. I’m still inclined to suspect the fuel delivery.

I know you tested the pump in a static condition, with the engine off, but I have to wonder if the pump is capable of supplying constant adequate pressure when there’s a constant draw on the supply, and when the “head pressure” is low due to low gas level or due to fluid dynamics inside the tank and around the baffles when making a tight turn, such as turning into a parking garage.

@thesamemountainbike It would be nice if I had a way to monitor the fuel pressure while I was driving. It hasn’t stalled recently, but the engine speed is still dropping and then rising again when coasting at around 25 mph, and it happens even when the tank is full and I’m driving straight. Also, I have no stumbling or hesitation at all when accelerating or coasting at highway speeds, for example.

Part of the difficulty I’m having is that I’m not sure what is the symptom and what is the problem. For example, is something (such as fuel delivery) causing the engine speed to drop and thereby causing the voltage to drop simultaneously, or is the voltage dropping and causing the engine speed to drop?

Also, thinking back on some of the symptoms I had been experiencing but ignoring (such as feeling the steering tighten around turns as it does when the engine speed drops), I’m pretty sure that the issue with the engine speed has been happening for a little while, but I didn’t look into it until the car stalled a few days in a row.

The only other thing I can think of is that when I replaced the fuel pressure regulator (with a brand new one) sometime within the last year, the vacuum hose didn’t seem to fit on the new one quite as tight as it fit on the old one. Perhaps when I’m coasting at 25 mph or less or in a parking lot there’s not enough vacuum to control the FPR properly?

You made good points Caddy. The fuel pump, like any DC motor, would se sensitive to a voltage drop.

You’ve also added a new clue. If the steering is tightening up, to my mind it adds the possibility that the power steering pump is binding under load and loading down the crankshaft, causing a drop in rpm and the secondary and tertiary symptoms.

Perhaps if you tried bypassing the PS pump with a shorter belt as a diagnostic test perhaps you could verify or eliminate it as a suspect. A seamstress’ measuring tape ($1 at WalMart) makes a great tool for measuring for a shorter belt. The steering would be hard, but the test could answer the question.

I thought about the possibility of the PS pump being an issue, but I also figured that since the steering is difficult when the car stalls, the steering tightening could be the symptom of the engine speed/voltage dropping rather than the cause.

Also, one of the most certain ways to duplicate the problem to coast between 20-25 mph on the straight, slightly downhill road on my way home.

A couple more updates.

I noticed that the issue seems to be worse when I’m braking rather than just coasting, but it does happen both times. One time that it never happens is when I have my foot on the accelerator. Also, perhaps entirely unrelated, but after I run it wide open (if have to when I’m merging or something), when I come to a stop at a light, the car will be shuddering/shaking at idle. This will go away after I have driven it for about 45 minutes to an hour after that.

So far I haven’t stalled over the past few days, but I would still like to take care of the issue. Someone suggested a vacuum leak, so I guess I’ll have to find all my vacuum hoses and listen carefully.

Hopefully the additional details that I posted have given someone some ideas :).

Well, the way you explain it now sure does sound like a vacume issue. I’d check all the hoses and nip the ends off just to freshen up the connection, but I wonder if a failed power brake booster could be doing this. Or a sticking idle air control or did you replace that already? Maybe someone will chime in.

@Bing I replaced the IAC with a used one which I cleaned. Although it is possible that two from different cars could be bad, I want to explore other possibilities before I buy a new one.

Yesterday, when I went to replace the power brake booster check valve (and found out that the universal one doesn’t fit), there was still quite a bit of vacuum left when I pulled it off of the booster after the engine had been stopped for 30 minutes. I then started the car and plugged the end of it with my thumb, and there was a lot of vacuum. I also didn’t hear any leaks, and the car didn’t seem to run differently with it off of the brake booster.

Could the throttle position sensor be worn out near the “closed” position and not sending the proper signal to the computer to tell the IAC that the throttle is closed? If this were the case, would there be any other symptoms that I am not currently experiencing?

Well you’re asking the wrong guy. I’m at the limit of what I know at this point. I thought Tester or someone had mentioned a problem with a brake booster once. You should be able to hook an ohm meter on the TPS and if you get a smooth reading all the way through its limits it should be good. If there are dead spots then it would be a problem. I can’t see a TPS just shutting the engine down though like that. More likely you’d get a stumble or something.

Are you getting any new codes though after clearing out all of the old stuff?

@Bing, thanks for your help. I was just putting the information out there in case someone might happen to see it and know what is going on.

I was thinking that perhaps there was a bad spot on the TPS at the lower end of the range which was causing it to tell the IAC that it was still partially open so it would not provide enough air to the engine. I’m not sure if this is logical or how it would explain the fact that there are little or no variations when the car is not moving and idling.

I’ll check the codes again and post them here. I appreciate everyone who has tried to help; perhaps it is not something that can be diagnosed from a distance.

@98caddy do you have a Fluke 88 or a Fluke 87? If you do, it has a minimum/maximum feature. You can backprobe the tps signal pin, set it up to record min/max and sweep the throttle. Minimum might be 0.5v, while maximum might be 4.5v. Then check your recorded values. If there is a 0.0V or 0L, that would mean open circuit (in other words, a dead spot).

@db4690 I wish I did!

I do have a Craftsman digital multimeter; I’ll find the model number and post it here.

if it has a min/max feature, so much the better. If not, you’ll have to sweep the throttle very slowly while watching the multimeter display.

Yesterday as I was driving in a town where the speed limit is 25 MPH in most places, it stalled on me three times.

After this happened, I added fuel injector cleaner and swapped the throttle position sensor with a spare, but the RPM dip keeps happening. I was driving on a slight downhill road at around 25-30 MPH to test it and I could pretty much time the RPM drop:

1000 RPM - three seconds - dip to 500 RPM and recover to 1000 RPM - three seconds - dip to 500 RPM and recover to 1000 RPM, etc.

What I can’t figure out is why it never happens if I have my foot even slightly pressed on the accelerator, and why it accelerates so smoothly without a hitch. I don’t even know where to look next.

So I changed my crankshaft position sensors (brand new AC Delco) and it didn’t do anything at all to fix the problem.

I was trying to figure out if there was any detail which I hadn’t posted yet. When I am driving and I cancel the cruise control with the lever, I hear somewhat of a “thunk” sound. Also, when I see the RPMs on the way down, once they get to 500 or below, if I hit the accelerator to try to “catch” it, it will hesitate for a moment with no change in the RPMs, then “wake up” with a jerk like I hit the gas hard from a stop.

I FIGURED IT OUT!

Last Friday, I found an old post on a forum somewhere about someone having the same issue. They mentioned that they had cleaned the EGR valve, so I figured with two bolts and a connector being all that needs to come off to remove it, I might as well try.

I held the valve in basically the same position that it is installed (meaning I didn’t turn it upside down) and sprayed in the opening with throttle body cleaner (not carb cleaner). I then used a “pipe cleaner” to clean the inside, and I also worked the plunger up and down after I sprayed it.

I had a few “false alarms” in the past thinking it was fixed, so I wanted to wait until I was sure before posting. I now have to lose the habit of constantly glancing at the tach to see if it’s about to stall.

Thank you to everyone who posted here to help me. It would be interesting to know why cleaning the EGR valve fixed it.

A defective/malfunctioning EGR will have a big impact on driveability, rough idle, stalling, etc.

Some Ford cars/trucks were “notorious” for having EGR problems.

Thanks for posting the update, and glad you finally got it solved! A faulty EGR – in its normal failure mode of allowing too much exhaust gas to pass into the intake manifold or allowing exhaust gas to pass when it shouldn’t be letting any through – that usually produces stalling-like (or hesitation) symptoms on acceleration, which you said you didn’t have. But if the valve sticks slighly open and never fully closes, that could produce the symptom you noted. That isn’t as common of a failure mode, but as you have found it, it does happen. Usually the ECM would detect it as an EGR failure, or at least complain about the air/fuel mixture beingout of whack, but for some reason your car’s ECM wasn’t able to figure it out in your case.

...allowing exhaust gas to pass when it shouldn't be letting any through...

I have that problem myself sometimes :).

But seriously, the car didn’t have any issues at all as long as my foot was on the accelerator.

I should have cleaned the EGR valve first, but you know what they say about hindsight. At least I only spent about $135.00 in unnecessary repairs. I should be happy I didn’t spend a couple thousand like some have before finding the issue.

Thank heavens. Very interesting. Now I wonder if thats what was wrong with my Riviera before I junked it. I know I looked at it but don’t think I ever took it off because it looked so rusted on that thought I would create more problems. You woulda thunk that there’d be a trouble code or something that would have been set. Who cares if it worked. Congrats. Now you’ve earned your star so you can post to help others here.

Thank you, Bing. I feel like I’ve earned a dope slap as well for not trying it sooner :).