Stall When Coasting Between 20 -35 MPH – [SOLVED!]

[Paul Huseonica]

Car:

1998 Cadillac Seville STS, about 200,000 miles

Symptoms:

When I’m coasting or braking, usually between 20-35 MPH, the RPMs will randomly drop from about 1000 to 400 or less, then either recover quickly, or the car will stall. The car usually starts right back up, but a couple of times it would start and then die immediately. It seems to be somewhat worse when making a sharp or long turn (like a downhill entrance ramp).

It never happens when I have my foot on the accelerator, and it doesn’t happen when I’m stopped at a light or parked and idling. There is no hesitation, bucking, or jerking when I am accelerating.

What I have done so far:

-Replaced the battery
-Had the alternator tested (came back as being good)
-Replaced the spark plugs with AC Delco brand plugs
-Replaced the crankshaft position sensors with AC Delco brand sensors
-Replaced the camshaft sensor
-Cleaned the throttle body
-Replaced the IAC valve with a used one from another car and cleaned it
-Replaced the throttle position sensor with a used one from another car
-Checked the fuel pressure at the rail and came up with 45-50 steady PSI (car off, key in ON position, released pressure and repeated 3 times)
-Checked the transmission fluid level (used proper procedure with transmission warm/engine idling. etc. Just below top line of “Full hot”.)
-Checked the power steering fluid level (added some)

I have pulled the following codes:

AMP:

-Current: NONE
-History: U1064 (Loss of Communications with DIM)

IPC:

-Current: NONE
-History: B1652 (Loss of Keep Alive Memory (KAM)), B1983 (Device Power Circuit Low)

IPM:

-Current: NONE
-History: B1344 (HeaterDefrost/AC Door Movement Fault), B1652 (Loss of Keep Alive Memory (KAM))

PCM:

-Current: P0135 (Heated Oxygen Sensor (HO2S) Heater Performance Bank 1 Sensor 1), P0147 (Heated Oxygen Sensor (HO2S) Heater Performance Bank 1 Sensor 3)
-History: P0134 (Heated Oxygen Sensor (HO2S) Circuit Insufficient Activity Bank 1 Sensor 1), P0603 (Control Module Long Term Memory Reset), P1599 (Engine Stall or Near Stall Detected)

RSS:

-Current: C1780 (Loss of Steering Position Signal)
-History: C1715 (Right Front Damper Actuator Short Circuit to Battery)

I’m at my wit’s end as to what to test next. Any help is greatly appreciated.

—————————————————————————————————
UPDATE 03/05/2013:

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.

Topic

[Wayne]

I’d take care of the O2 sensor issues first, and go over the exhaust in general.

Then I’d check for vacuum leaks.

Then I’d clean the MAF sensor with MAF cleaner, see if it helps at all. This is a pretty common issue. Can be due to a few things, TB is one of them, which you’ve ruled out. The most common is a vacuum leak. Cracked hose, bit of air turbulence coasting and badda-boom-badda-bing, fluctuating idle issue.

Anything from a dirty MAF sensor to a bad PCM can account for it, you’d have to eliminate these one by one.

[Paul Huseonica]

Thank you, Wayne.

I forgot to add that I did clean the MAF sensor (with MAF sensor spray cleaner) when I took it off to clean the throttle body. I have listened for vacuum leaks, but I did not use the procedure in the video. I’ll do that and post an update.

[Wayne]

Sorry was in the process of editing when you replied; really should solve the basic O2 sensor codes first. The KAM, communication loss, and such I wouldn’t worry about unless you never unhooked the battery.

Quick resistance test of that first sensor heater circuit should do to verify. If they’ve never been changed, you may wish to just go ahead and do all 3 (or 4) with new OEM ones since its usually a good idea to do them around 100k or so anyhow.

[Paul Huseonica]

I’ll do the resistance test as well. I didn’t do anything with them because I thought the oxygen sensor heater was only for emissions purposes when the car was cold, and I have had those codes for a long time before this new problem developed. I wasn’t aware that it could cause my RPM fluctuation/stalling issues.

Do you know what the resistance values should be for the O2 sensors, and which “prongs” I should test it between on the plug?

[Wayne]

The o2 sensors may or may not be related. One way to tell, take it out quick to check when the car is stone cold, say overnight, take it out in the morning, do the coasting bit again before the o2 sensors go into closed loop. If everything is fine, then it might be due to them, or an exhaust leak giving that/those sensor(s) faulty readings.

See around the 4min mark in the vid below to see what to look for to verify heater circuit o2 sensor failure. Be sure to get OEM, AC Delco I believe, don’t just hit autozone for them. http://rockauto.com for example would probably be a decent place to get them from online.

[Paul Huseonica]

That’s another detail I forgot to post – it doesn’t happen until the car warms up, and it seems like it might get worse when I make a bunch of short trips after it is warmed up (post office, a few miles to grocery store, another few miles to the bank, etc.).

Some people told me that the gas mileage would drop a huge amount if the O2 sensors were bad, but that hasn’t happened.

[Wayne]

The gas mileage will drop significantly “most” times, but not always, and other times, like if you normally drive it like you stole it, you may not notice it at all since it’s forced into open loop for the majority of your road time. 😉

It’s just a good idea to take care of what’s being reported as already wrong first, before moving on. And this is a maintenance item that needs to be done anyhow.

[Paul Huseonica]

Thank you. I’ll post an update on what happens when I change them. Is there a preferred method to checking for exhaust leaks (besides obvious ones that you can hear easily)? I checked Eric’s video page but I didn’t see a video on exhaust leaks.

I also forgot to add that my model/year of car has frequent problems with the front motor mount since they didn’t supply any torque struts with the engine, so there is a good chance it is broken. I don’t know if that is helpful or confuses the issue, but I figure that when someone is taking time out of their day to help me, the least I can do is to give them all the info that they might need ;).

[Wayne]

I’d probably go over the exhaust manifold(s) first thing.

[Paul Huseonica]

I knew I missed the video somewhere! Thank you, I’ll be sure to update.

[Paul Huseonica]

I tried my best to inspect the exhaust system today, and I included a few photos. The exhaust manifolds can’t be seen from above the car, and are difficult to see from below. Below are a few photos of some parts of the system in case anyone wants to point out anything about it.

I also took a video of my car idling (not completely warmed up). It seems to idle pretty smoothly; when I get a chance, I’ll try to post a video of the dash when I’m coasting around 25 MPH.

Attachments:

• exhaust_001_forum.jpg
• exhaust_002_forum.jpg
• exhaust_003_forum.jpg
• exhaust_004_forum.jpg

[Wayne]

Showing it stalling really isn’t going to help.

I see a couple suspect spots already (big one with the last 2 pics), but there’s no way to picture/video quarterback it, you’ll really have to stuff a rag in and listen and hand-hover for any exhaust leaks you suspect. Sorry man. 😉

[college man]

If the exhaust leaks are before the o2 sensors.
That will cause all kinds of problems.since its
cold out.due the exhaust test first thing in the
morning.when you start the car and put the rag over
the outlet of the exhaust.You will see the smoke that
normally would be coming out the back coming out any
holes.By the looks from those pictures.There are quite
a few.

[Paul Huseonica]

Thank you for the replies. I guess I’ll have to find someone to help me hold a rag over both sides of the dual exhaust while I get under the car.

[Wayne]

Sheesh, what kinda redneck are ya, no duct tape? That’s just unthinkable… 😉

[Author]

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