Once the car heats up, its like a vac leak opens up. Like a solenoid fails. Or a seal fails.

I had extreme throttle body shaft bushing wear and i just finished rebushing the shaft to as new tightness so that vac leak is gone.

I am now able to get the IAC to 35 (tunerpro rt) and it will hold the target idle of 700. Perfect.

the problem is once we enter closed loop, ....when i goose the throttle a couple times its like it suddenly initiates a vac leak and i cant stop it. the IAC closes to 0 to try to bring it down, but i end up at 1300rpm. when this occurs, the o2 starts reading 800mv rich steady, and blms are 148.

fuel pressure is about 45. of note is that it is an adjustable type. i havnt messed with it because fuel pressure seems fine. injectors are good, all ohm16 hot and cold, all were cleaned and reinstalled when i did the plenum.

tomorrow i will try to clamp off every vac line to rule them and things like charcoal cannisters and vacuum accumulators solenoids out.


ive pulled the plenum apart and reassembled with good sealent on the gaskets. its not that. i tried propane, and carb cleaner. when the plenum was off, i pulled the egr and tested it, its fine.

i cannot hear a vac leak, which you would think i should be able to hear the his.

the ecu was fubar. rusty. the prom is a custom from zimmerman. it was corroded too. i cleaned it up, and put in a 1227730 ecu and the vehicle is acting the exact same.

rebushing the tb corrected the inability to set the idle, but there is something very strange going on that sends the idle very high and its been doing this since the day i bought it. none if the work ive done has affected this high idle condition.

also note: when i let the car idle with the electronic spark advance disconnected (so running on base idle) it will not runaway up to 1300rpm. maybe that isolates it to closed loop?

if u can help brainstorm id appreciate the help.

 

put a factory prom on the car. no difference

its clear there is a mammoth vac leak.

With the throttle blade fully closed, iac commanded to position zero, she idles about 650

is it possible to have an intake manifold gasket that leaks air into the system?


anybody else had any big but hard to find vac leaks?

 

smoke the engine to isolate the (possible multiple ?? ) leaks , best and quickest way to solve, IMHO

 

to find a vacuum leak, try using a smoke machine.
There's guys on youtube that show how to build it DIY cheap or buy one. Scotty has an old vid where he spent $100 and it works.

Perhaps the EGR valve on top of the intake is letting air in.
fwiw I've disconnected the EGR solenoid vac line, I removed it actually, and disconnected the cannister vac line, well the hard line cracked same thing, didn't even bother to plug it or anything, and engine still runs fine.

 

every vac port on the plenum and tb has been plugged in an attempt to find the leak.

rebushed the tb

drenched the intake manifold base with water to see if there is slurping....nope


i have a cigar.....i could do the smoke test.....but im not going to find it that way. its not a typical vac leak.



some more info:

there is a major correlation between heat and this vac leak.
furthermore, when i command the iac closed by shorting a+b, then pulling power to keep it there.....im getting air going through the iac channel still. when i plug the iac passage way it with a rag i can lower the idle but it still will not die.

when i put a piece of a vinyl binder over the intake it will die.

if feels to me like the issue is the tb itself but i cant figure out what.

 

@ihatebarkingdogs clamped the booster line off no dice.

is it possible to have a gigantic leak in the intake manifold base?

internally?

 

I can't see how there would be an intermittent heat-related leak in the manifold base. There are those large hex plugs underneath inside the oil splash stamped steel cover bolted to the underside of the base, but if any of those were leaking air / oil fumes into the intake stream via the EGR ports you'd think it would be all the time, not intermittent. Plus they're torqued in there with anti-seize pretty tight usually.

What about your PCV hoses and valve? Is there a hole or large crack in any of the hose / valve / fittings that could open up under heat load? I know you wrote that you've plugged up every port in an attempt to find the leak so I'm just spit-balling here....Maybe a gasket failure in the throttle body plate up top or below on the IAC housing that opens up under heat expansion?

 

I capitulated and did a smoke test. I knew it wasnt a hose or anything like that and it wasnt. but it was still useful to do because it demonstrated HOW BAD THE THROTTLE BODY IS. When i taped off the tb and blew in brakebooster vacc line it comes out the hinges still very badly. I did re-bush it, and did a good job doing it too actually with zero play in the shaft. But the smoke still comes out there. I have to blow to make it happen but it comes out. That said, i dont think the air is coming in through the outside of the tb through the hinges. Rather, i think its sneaking around the outside of the tb plate, mostly on the driverside, inside the throttle body where the little factory V notches are. see pics below.


dogs, ive never heard that there is more than one pintle shape! that would explain alot actually. i found what you said mentioned here as well. I have 3 IAC’s here all from early Tpi’s and i will see if there is ang pintle shape difference among the three and take them into the local parts store and compare to one that comes up for a 90 L98.

https://www.corvetteforum.com/forums...post1601917045


Maybe u guys have comments about this: I put playdough in the v shaped notches of the tb plates. It killed the engine. So there is enough air getting around the outside of the tb plates at the hinges that the car can run with the IAC closed, and the tb plates fully retracted. (there is a gap between the tb plate adjustmment screw and the levers foot.

In order to start the engine like this, i do have to open the tb plates a tad to get it started. But once the engine is warmed up a tad, it will run at about 650 rpm. wth?? Shouldnt the engine be starving for air like this? If i let the engine get nice and warm, the engine will idle at 1300 like this!!!!

Obviously there is something significantly wrong with the amount of air sneaking past the outside of tbs through the V notches.

when i used some of the kids play dough in the notches, it killed the engine. There is no visible play in the hinge bushing. (after i rebushed it).

pics:




u can see some residual playdough that got sucked in there. in there. i did remove that with a pic and a vacuum.

here is a bbk 52, i was planning on using this TB but the cable linkage is different. this is from an early tpi. mine is a 90. I have this up for trade to a late tpi bbk but at this point im ready to grind off the linkage retaining peens and take the late tpi throttle body linkage and put it on this.

edit: i think the iac passageway is different on these early tbs too.

 

I think the plate notches are there specifically to let some air around the hinge as part of the normal idle air supply (beyond what the IAC provides) - otherwise the disk would be fully circular and that makes sense. But the issue with the false air that you mention is still very real, whether there's more air coming in through the plate axle / hinge / bushing / elsewhere in the TB. Since you have the other BBK handy, you could just run a test install swapping it in (without the full linkage mods, etc.) and see if the idle still surges. At least then you know it's the OEM TB and not something else. But I think you're pretty much there now...

I like what Dogs pointed out about the IAC pintle shape, which also reminds me that the pintle seat itself sometimes gets gunked up with carbon deposits. But you're on top of everything else so I didn't think that was a probable cause. Nonetheless, it's a good idea to check the pintle shape / seat match....

 

Yeah i do agree that its obvious the V notches are there on purpose....and its possible its not them that is the issue. It could still be the IAC not providing a full seat.

In fact im going to re-do the test, and ill put playdough in the iac passageway. If the idle comes down then id say its likely a matter if the iac not seating properly.

Do we know ...if a fully seated IAC should allow zero air past it?


edit, i can also verify that the iac and the bore/seat is clean as a whistle. Did that along time ago.

 

I looked at this just now and actually it will not work without me machining/drilling holes. The IAC air passageway on mating surface /rear tb to front plenum / is different on the early TB’s.

too much modification and i become a bubba. i would just find a new tb.

 

Yeah, I'm with you on that.

Just a thought; when you re-bushed the TB axle you applied a lightweight oil into the bushing areas, working the axle back and forth to pull the oil in right? Still scratching my head trying ideas on the false air intake source....

 

yes i did do that with the bushing. also i sprayed it down with fluid film.

the more i think about it, there is simply no way that the throttle body bushings are causing a massive leak. they are TIGHT after i rebushed it.



Question, anyone know how the IAC pintle extension works? Does the stepper motor simply keep turning the gear and the pintle eventually meets resistance when it hits the cone shaped bore? or is it an actual “count” or voltage that the motor extends to?

Im wondering if the exu commands the IAC closed, its shows me on tunerpro that its commanding 0 counts.....how do i know if the pintle is firmly pressed into the cone shaped bore? what if 0 counts is actually not fully closed?

anyone know how to test this? looking in the fsm now.

 

The IAC is calibrated after initial installation / ECM reset by the ECM commanding it to full closed when it meets resistance against the pintle seat, then from there it assumes all commands to the IAC are executed using those counts (extend / retract signals). So if it's off the seat and something happens (like you install a different thickness gasket or the IAC starts backing itself out of the mounting threads) it'll change the amount of air. But since you mentioned that this happens only when the engine is up to operating temp it sounds more like an intermittent failure of the IAC or the false air that you've already been chasing.

 

Thanks - that's a much better and clearer explanation than the one I gave. Appreciate it.

 

look closely. u r looking down the bore of the iac pintle seat. im swapping the iac housing right now

 

so, im really not sure wtf is going on with that iac passage bore.....but it is oversized. it doesnt look like it was worked or ported.

maybe this car had a slightly bigger sized pintle from the factory?

But i took the iac passageway from the bbk and installed on the one that *looks* like the factory unit and problem solved. the iac can now make a good seal (for the record, its not a perfect seal, vacuum will build and accumulate if you put playdough into the iac channel. it does get sucked in.

But that bore was just a hair too big, and air was sucking past the pintle, even at “0” counts on the IAC. which meant i could not get control over the idle.

This felt like i was chasing a damned ghost. glad this one is behind me.

for those of you with play in your tb, its probably not a huge issue. mine was obviously this weirdly oversized bore. i probably didnt have to re-bush my throttle body.

thnx for the brainstorm everyone.

 

Well I see those two IAC housings are different part numbers - so that could be the whole deal right there. If the IAC valve was designed for one, but the other indeed has a bigger bore that wouldn't allow a full seat, I think you've just nailed your problem as you mentioned.

The lower pic has had the IAC housing bolts swapped out from OEM Torx head to the Allen head style, so that's a pretty good clue also that the housing may have been swapped previously...

Props to you for finally figuring it out. They're not easy sometimes, especially since if not the original owner we don't know what was done to the car beforehand.

Cheers,
AJ

 

exceptional result, good diagnostics along with problem solving, thoughtful tenacity is always a benefit !! Stay Well

 

dogs, do u have any part numbers or references here? i think this tb had a different iac pintle shape. the orifice doesn’t look like anyone ir anything increased its diameter. looks like it was built this way