well.. as the title says.. Been talking about this for a bit but i can be more specific now..

The car runs great except for this and the idle fluctuates (bounces slightly) for a bit then smooths out.. But whenever i take my car on the highway (or any road where i can do 100+km/h) it runs great.. nice and quiet... but almost everytime i come off the highway or decelerate to a stop.. the car dies..
it doesnt chug or shake... its like it turned itself off... and starts back up fine..

Now at first i figured it was the TCC sticking.. But just 30min ago.. it did it again... But it didnt do it at the first stop.. i came off the highway, stopped at the sign for a second then drove on.. i get to the light a few hundred feet away... i have a feeling the car is going to stall so i throw it in neutral... and it still died.. so not the tcc..

any ideas on what this could possibly be? like i said, my car hasnt had a single CEL/DTC since i owned it but it seems to run lean at ldle.. (i have my own scanner)
short term fuel trim shows at worst -18% at ldle.. but will go back to 0-3% once i start moving...

i need a gas tank so that and the fuel filter will be replaced once i have the cash.. but i dont think a fuel filter would cause this specific a problem.. and the pump works fine under load so i dont think thats it..
I also plan on cleaning the TB and TPS sensor once i can feel my hands outside again.. lol.. anything else i should check?

99 2.4 / 4t40e 158k km

I would clean the IAC or replace it. It is responsible for idle control.

actually all sensors will get removed and cleaned up.. but i dont want to replace anything without trying to figure out what would cause such an isolated problem..

if i drive around all day at 80, there is no problem.. its only after hitting a certain speed that this happens..

im not so much even looking for a solution, im looking for a possible cause... what could cause a fine running car to stall after highway runs but nothing else?

i mean sure (and i appreciate any feedback here), i could sit and rattle off TPS, IAC, vac leak, FPR and the dozen or so other things everyone always suggests.. and one of those may be correct, but i would like to learn what is physically happening and why.. the mechanics of it.. it would actually frustrate me to fix this and not know why it happened in the first place.. lol.. im just like that..

for instance.. i doubt a bad pump or filter would do this as they would act up under load.. not just one specific moment.. same with timing, it would only get worse as RPMs increase.. my car doesnt have an EGR valve..

IAC does sound like a likely cause.. but do you know whats its actually doing and why it would shut my car off at that specific moment?

its like in math class.. you gotta shown your work! haha... even if your answer is correct, i wanna see how you got there..

ok.. i've been checking the scanner again.. and doing some reading... my understanding of STFT and LTFT was incorrect.. which is pointing me in different direction..

At 600 rpm idle.. my car shows 25% load and -18% STFT... now i always thought this to mean my car was running lean.. however it means the ECU is leaning my car out because it perceives a rich condition.. so quite the opposite!! lol...

so im rich at idle, but lean out as i drive... bad FPR or leaky injector maybe? but im thinking now the O2 sensor could be the main cause of this... a malfunctioning o2 could cause enough havoc on the fuel trims to cut my car off after a hard run... correct? would be nice if something would throw a code!!!! lol

now the 25% engine load at idle doesnt seem right to me, but i cant really find a reference anywhere...

well.. now i've stumbled on this article on the map sensor... and it gives me hope!

now the MAP sensor could be bad and account for the rough idle and rich condition.. i suppose the o2 sensor could still have issues..but this last part just clicked!

Quote:

A vacuum leak will reduce intake vacuum and cause the MAP sensor to indicate a higher than normal load on the engine. The computer will try to compensate by richening the fuel mixture and retarding timing -- which hurts fuel economy, performance and emissions.

that accounts for the 25% load at idle and the rich condition.. but more specifically, it accounts for everything else..
by it running so rich at idle due to the vacuum leak, the ecu tries to lean it out by adjusting fuel trim.. but once coming off the highway and vacuum builds (then leaks), the fuel trims are now incorrect as well as the timing so the car dies before the ecu evens things out again.. would also explain why it starts right back up everytime.. its reset itself by then...

sound about right? this actually makes sense to me as i can chain it all together so i have hopes its a cheap fix!! lol

think i found it.. lower o-ring on at least one injector seems to be leaking.. poped the hood right after getting home and the base of the injectors was soaked.. that would account for the vac leak, correct?

also noticed it did it to me just cruising around the city so its not after a highway run, apparently its just once the car really warms up.. the highway was only coincidence..
makes sense...

will change out the o-rings on all of them and see if that helps..

well... just replaced the gas tank, fuel lines and filter...it all needed to be done... blah.. car def. runs better, but the problem persists... but at least now is a bit more localized..

Whenever im driving along and take my foot off the gas completely , once i give it gas again, the car wants to stall unless i go at least half throttle, like the pedal does nothing to RPMs until that point, they just keep falling... and it only happens once the car is warm and even then not all the time..

Could this just be a matter of a dirty throttle body? car runs totally fine other then in that one situation..

My mechanic kinda ran out of time (and i ran out of money.. lol) but i now have a schrader valve at the fuel filter (stupid GM) but wasnt able to get pressure numbers as of yet.. i kinda suspected a bad fpr all along so thats still on the to-do list, just wonder if anyone has an educated guess for me.. lol