Random idle stall at stop in gear
#1
02-28-2019, 11:36 PM
Random idle stall at stop in gear
1998 Blazer, 4x4 200k miles. Once warmed up under normal driving when coming to stop (stop lights/ stop signs etc) at random times engine dies. Starts right back up no problem, no engine codes on the scanner. Fuel pressure check- good. So, normal idle this truck in park is at apx 650rpm no load (air off, lights off etc). Put into gear and drops some to 600, pretty low is my thought for something with 200k miles. Recently replaced idle air valve because I’ve seen this type of thing in past with a Dodge. No joy. TPS replaced last week when service eng light came on and I noted throttle acting funny (rpm not drop off when coasting like it should). During all of this the idle issue remained. So I figured turn up the base idle screw (torx) and now its at 700-725 in park no load but when I put into gear still dropping to low 600’s. Thoughts? Vacuum leak maybe but not seeing anything else wrong if there was one? The DIY did not have anything on idle settings. Web search vague at best but seems I’m still in ok range. Rest of driving performance as expected, pulls strong, no hiccups and shifting normal just the random die when stopped/almost to stop I guess (darn thing so quite to my old ears sometimes I don’t notice its already shut down because I’m watching the traffic and not the dash indicators).
I do not know what other information to add, my normal motto is if it ain’t broke don’t fix it so nothing else engine wise recently been changed other than normal maintenance.
I do not know what other information to add, my normal motto is if it ain’t broke don’t fix it so nothing else engine wise recently been changed other than normal maintenance.
#2
03-01-2019, 12:55 PM
Idle is fully controlled by the PCM and manual adjustment of the 'base idle' should not be attempted.
The most common problem that I have seen here is a gunked up EGR valve that is partially stuck open and allowing exhaust gases back into the intake when it shouldn't be. There really isn't much you can do to diagnose this other than attempt to command it open with a scan tool and observe the fuel trims. If you have a scanner capable of bi-directional control, you can do this. If not, just go pick up a new EGR gasket and some sensor safe carbon cleaner then remove the valve & give it a thorough cleaning. That is typically all that it needs. There also should be a gasket available that has an integrated screen on the intake side of the valve that is supposed to reduce the likelihood of this problem happening. Basically, what happens is that carbon will build up inside the rather long EGR supply tube and then chunk off. This happens more in older vehicles that are not running optimally (poor combustion). More often than not, these little bits of carbon do no harm, but every so often one will get caught directly in the pintle of the EGR valve, holding it slightly open. If that happens, unmetered exhaust gas is introduced into the intake and displaces combustion supporting oxygen. At idle, in gear is the worst time for this to happen and the engine can stall as a result.
As you mentioned, a vacuum leak could cause a stumble/stall. I have successfully located vacuum leaks using an unlit propane torch tracing along the vacuum lines with the engine running. If you note a change in the sound of the engine as you pass over a line or joint, then you have a problem at that location.
The most common problem that I have seen here is a gunked up EGR valve that is partially stuck open and allowing exhaust gases back into the intake when it shouldn't be. There really isn't much you can do to diagnose this other than attempt to command it open with a scan tool and observe the fuel trims. If you have a scanner capable of bi-directional control, you can do this. If not, just go pick up a new EGR gasket and some sensor safe carbon cleaner then remove the valve & give it a thorough cleaning. That is typically all that it needs. There also should be a gasket available that has an integrated screen on the intake side of the valve that is supposed to reduce the likelihood of this problem happening. Basically, what happens is that carbon will build up inside the rather long EGR supply tube and then chunk off. This happens more in older vehicles that are not running optimally (poor combustion). More often than not, these little bits of carbon do no harm, but every so often one will get caught directly in the pintle of the EGR valve, holding it slightly open. If that happens, unmetered exhaust gas is introduced into the intake and displaces combustion supporting oxygen. At idle, in gear is the worst time for this to happen and the engine can stall as a result.
As you mentioned, a vacuum leak could cause a stumble/stall. I have successfully located vacuum leaks using an unlit propane torch tracing along the vacuum lines with the engine running. If you note a change in the sound of the engine as you pass over a line or joint, then you have a problem at that location.
#3
03-01-2019, 01:16 PM
Oh thank you, why did that not occur to me. At 200k miles I noted quite a bit of carbon build up in throttle body as is when I replaced the TPS and cleaned that up. WIll get the gasket and cleaner and report back. Prices I saw for the EGR cleaning is the way to go for sure!
#4
03-01-2019, 01:30 PM
Cleaning the EGR valve is VERY cheap and almost should be done as routine maintenance on these vehicles as it is also VERY easy to do. An electrical connection & two bolts is all that it takes to get the valve off. I would also recommend quickly starting the engine with the valve off & the intake port plugged off with something that isn't going to get sucked in. This can allow carbon in the supply tube to be blown out more or less harmlessly before you put everything back together.
#5
03-01-2019, 08:25 PM
Haha, what a pain. Is anything on this truck easy to do? Yea, removal of EGR easy but that gasket material was not. Did not want crude falling into port so used a shop vac while getting the old gasket off plus some of the carbon build up was a chore. That valve was carboned up bad, cleaned and smooth action now. Port cleared out, gasket on. It’s late here, will test drive Saturday and report result but think that should solve it. Again, thank you very much.
One of my other issues to tackle and I know what has to be done but am reluctant to even do it yet- fuel tank gasket ring leaks when full tank. Dropping tank, replace and I’m like if I got to do that might as well change pump but hate fixing what is not broke. Last is just adding keyless entry. It’s an LS, pretty much loaded but no keyless module. Figure go aftermarket only because if I read right on forum the BCM needs reprogramming to support factory keyless which makes aftermarket way cheaper or is this info wrong?
One of my other issues to tackle and I know what has to be done but am reluctant to even do it yet- fuel tank gasket ring leaks when full tank. Dropping tank, replace and I’m like if I got to do that might as well change pump but hate fixing what is not broke. Last is just adding keyless entry. It’s an LS, pretty much loaded but no keyless module. Figure go aftermarket only because if I read right on forum the BCM needs reprogramming to support factory keyless which makes aftermarket way cheaper or is this info wrong?
#6
03-02-2019, 02:18 PM
ARGG! Ok the update. Went to take out of garage, engine ran so so bad, idle bouncing all over, low end just shaking and struggling to stay running. I was like what the heck, was better before. Tore it back apart, pull up testing egr valve. Basically every check was ok but..the 12v red line in. I have 12 at connector when unplugged from valve and resistance from A to E is 9 ohms but voltage drops to zip when plugged in. Very odd. Figured also does not look like it seals well but it should work tho not like it is currently running after cleaning it all up. Discovered the new gasket is not symmetrical and I had it backwards making a poor seal. (Bad eye sight) Reinstall and back to square one, stalls out and maybe a bit worse because doing it more often than random. Purchased a new EGR valve (costly buggers) and letting it cool off before I install this one. Oddly thru all of this still no dash light warnings nor codes on scanner. Crossing fingers this corrects problem, will update when done and retested.
#7
03-02-2019, 08:54 PM
I’m chasing my tail on this. New EGR installed, no longer stalling out but now when at speed a lot of the time (not always but mostly) the idle stays high like on cruise control and your fighting the engine to bring it to stop. Once stopped for a few secs it drops to normal- (low 600’s). Still nothing for dash idiot lights and no codes on scan. Acceleration is smooth and power with it. In last few days replaced idle control valve, TPS and EGR valve. Maybe, just maybe that fuel pump is flaky. I don’t know anymore, got a headache thinking about it. Open to ideas.
#8
03-03-2019, 03:09 PM
Well damn. That quick/easy repair kind of backfired. Did you attempt to clear out the exhaust gas supply tube?
The way the EGR valve is controlled is by a pulse width modulated signal from the PCM which monitors both the power & the ground currents as well as the position circuit in the valve. Your observation of having power and then not does not mean that the valve is faulty, but I see that you have already replaced it so I won't go into further detail here.
Do you have a scan tool capable of reading IAC counts? It sounds to me like you have a rather large vacuum leak somewhere now like you might have knocked a line off somewhere.
The fuel pump would not cause a racing idle.
The way the EGR valve is controlled is by a pulse width modulated signal from the PCM which monitors both the power & the ground currents as well as the position circuit in the valve. Your observation of having power and then not does not mean that the valve is faulty, but I see that you have already replaced it so I won't go into further detail here.
Do you have a scan tool capable of reading IAC counts? It sounds to me like you have a rather large vacuum leak somewhere now like you might have knocked a line off somewhere.
The fuel pump would not cause a racing idle.
#9
03-03-2019, 05:09 PM
Hi, yea I did clear it out, was plugged with carbon. Got desperate so called my baby sister’s hubby. He was a mechanic for years and owns a s10 pickup and a bit of a fondness for the s10 pickup/blazer line with the Vortec 6, just hard to get a hold of. Like you thinks vacuum leak. His best guess when I worked on cruise couple of those lines were in rough shape and moving things around some got them leaking bad enough to cause the problem..makes sense, said to plug them off, test drive. So I plugged line to fender, and took line that run back towards distributor and plugged it into other end of “t” leaving the rest there disconnected. It snowed here and figure not a good idea to test on slick roads while fighting the engine when stopping. Should be okay to try out Monday and will report results. If I note improvement will run to autoparts store and get some tubing, several pieces are suspect.
#10
03-05-2019, 01:53 PM
Lets have a “Tom Slick” cheer- yea No more stall at idle and idle settles like expected when driving and you let off accelerator pedal. Vacuum leak as final fix, was combo. Replaced Idle control valve, TPS sensor, EGR valve and gasket and port cleaned, new vacuum line pieces at the “T” split near replaced boneyard cruise control module. 200k causes lots of carbon build up (purchased used, not my doing). Have not like spent days testing but a 30 min road test passed. My view base idle still lower than I’d like (590-620 rpm in gear stopped) for a engine with over 200k on it but hey it’s working so leave it alone. My thanks for the feedback and ideas. Other stuff to work on is different section and not need help yet. Till then I be lurking around as I get it done. Side question, is there a way to tell if the injectors have been change up from spider to cpi type without tearing engine apart seeing I purchased used and have no clues from prior owner?
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