They look lean, bit of wear.
CK5
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TBI question
- Thread starter Chickntrk
- Start date
In my experience it would be odd for an injector to have problems when cold and not hot, but you have good fuel pressure and the O2 sensor seemingly isn't a factor when the issue happens. But there is evidence it's lean. Test for injectors here: https://www.thirdgen.org/forums/tbi/565900-t-b-i-injectors.html#post4489493
EGR is a bit of a mystery to me on these things,as it's supposed to be tested by the ECM on startup, but seemingly never triggers an EGR code when it's disconnected. See below...it's a "during" test. Without a code, I don't know if it will run lean at cruise or if the ECM is "smart" enough to add fuel like EGR isn't there if the EGR fails it's self-test on startup.
If it were a consistent ignition problem I wouldn't expect lean to be what you'd see on the plugs. I'd test the injectors, and check their pattern.
Datalog could be helpful, but I understand it's kind of a pain to get set up to record. All I can think of is you are lean enough to get into knock and timing is being pulled because of it. Still at a loss what changes when at cold WOT vs warm WOT that would mean lack of fuel.
Fueling changes based on temp (both closed and open loop) but even if injectors are the problem I'd expect to see it either only when hot, or regardless of temp. Not at a specific throttle position, only when cold.
EGR is a bit of a mystery to me on these things,
If it were a consistent ignition problem I wouldn't expect lean to be what you'd see on the plugs. I'd test the injectors, and check their pattern.
Datalog could be helpful, but I understand it's kind of a pain to get set up to record. All I can think of is you are lean enough to get into knock and timing is being pulled because of it. Still at a loss what changes when at cold WOT vs warm WOT that would mean lack of fuel.
Fueling changes based on temp (both closed and open loop) but even if injectors are the problem I'd expect to see it either only when hot, or regardless of temp. Not at a specific throttle position, only when cold.
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gmbellew
Registered Member
the ECM tests for EGR operation on a steady hwy cruise type of situation. that's when it will throw a code if it doesn't see it operating properly.
Good catch, I definitely mixed up some of the verbiage/tests in the manual for other events.
gmbellew
Registered Member
yeah, the knock sensor self tests under mild/hard acceleration. it's hard to keep it all straight!
Should have throw a code by now, its seen plenty of highway time in 6 years.
gmbellew
Registered Member
has somebody tuned the EGR out with a modified PROM? if not, then there is something amiss for sure. is it possible you can reconnect the EGR valve to the solenoid and have it functional again?
Not that I know of, I mean I really have no desire to have a functioning EGR.
It really doesn't seem like the self test seems to work that well on the EGR. Just anecdotal, but I recall a few people that have disabled EGR and never saw a code, and it appeared their systems were working properly otherwise.
Maybe they just had very unsteady throttle habits lol
So I couldn't get it out of my mind that this was somehow related to the TPS so I checked wiring and voltages again, voltage was normal but there was a bit of corrosion at the base of one of the terminals (like maybe something was corroded inside and pushing its way out). I got a new TPS and it was junk right out of the box, literally had no throttle at all with it. Returned it and got another and the issue seems to be resolved. I did also remove the EGR valve and made a block off plate which seemed to help but there was still a slight dead spot. TPS seems to have totally resolved it. I also got rid of every vacuum hose except the brake booster and MAP, so that cleaned things up nicely.
Mine is a 1988 with the 465 trans.
I leave all the origional junk hooked up- including the air injection.
I had a heck of a time getting it up and going a couple years ago (the blazer. I had a hard time getting THE BLAZER up and going after it sat for a while). After figuring out the spun balancer issue and getting the timing corrected, I was still having smaller issues and a hesitation similar to what you discribe- 1/2 way through the process I finally said screw it and hooked up the smoke machine- had quite a few vacuum leaks, including the the tbi throttle shaft. replaced all/every vacuum hose and fittings. Also swapped a factory TBI back on. the one with the loose shaft was bored and I would like to put it back… just need to get around to rebushing it or take it somewhere to have it down.
I also believe I had an exhaust back pressure issue that was fixed with a new muffler, but between that and new vacuum hoses, and new plug wires- was running like a new truck again
I leave all the origional junk hooked up- including the air injection.
I had a heck of a time getting it up and going a couple years ago (the blazer. I had a hard time getting THE BLAZER up and going after it sat for a while). After figuring out the spun balancer issue and getting the timing corrected, I was still having smaller issues and a hesitation similar to what you discribe- 1/2 way through the process I finally said screw it and hooked up the smoke machine- had quite a few vacuum leaks, including the the tbi throttle shaft. replaced all/every vacuum hose and fittings. Also swapped a factory TBI back on. the one with the loose shaft was bored and I would like to put it back… just need to get around to rebushing it or take it somewhere to have it down.
I also believe I had an exhaust back pressure issue that was fixed with a new muffler, but between that and new vacuum hoses, and new plug wires- was running like a new truck again
