Ape0r

Back story: I bought a 92 in July and have been slowly bringing it back to working order. Since I've had it, the engine has bogged down a bit when first put into gear (somewhat intermittent, may be tied to engine temp). I got an ALDL cable to check sensors, and everything checks out okay.

That got me after the idle. Warmed up, car wants to idle at ~600 rpm (desired idle from ecm), but idles around 950. IAC counts are zero. I have cleaned the IAC to my wits end, and finally changed it with no result. Also checked connection from ecm, which is fine.

Verified no vacuum leaks by capping all vacuum lines except PCV and FPR and running motor, no effect on condition. Carb sprayed everything of relevance: throttle body, intake, injectors, vacuum hoses (especially PCV and FPR since I ran it with them connected still). Did get a slight pick up in idle (950 to 975) by spraying egr, which I replaced with intake on car previously, but nothing significant.

This weekend I blew apart the throttle body and found nothing to indicate a leak. Also verified that IAC is sealing when shut by closing it in diagnostic mode and taking out connector to make it stay closed. I went so far as to fill the iac inlet in the coolant plate with water to make sure it was sealed, which it was.

After finding nothing in the throttle body, I proceeded to do intake gaskets "while I was in there." Intake gaskets looked relatively new, and were fel pro brand. I put in the same type. Only anomaly was some oil on one of the gasket faces, which makes me wonder if it is sucking air from the lifter valley. I also replaced the egr gasket.

No change in condition after changing intake gaskets. Throttle valves are closing nicely. Aside from a warped intake allowing it to suck air from the lifter valley, I am out of ideas.

Fuel injectors are new and were bought from FIC, Bosch 3's just like everyone else. Only possible issue I can find in the DataMaster readout is low Lterm counts (108 vs. 128, where it should be).

What do I do next? I can't see how it could idle high without getting extra air, but I've covered every base. As I said above, all I'm left with here is a possible warped intake/head. Car runs great otherwise. Only other current engine issue is the occasional rough hot start, hard to describe. Sounds like starter teeth not meshing correctly, and only happens when things are on the hot side (ie, doesn't happen after driving in rain, only after driving when dry out).

Ideas please!!! Will confirm hot idle is still ~950 when traffic dies down. Possible that the egr gasket may have dragged it down a bit, but it was still idling high (~850) with iac shut out in the driveway before fully warmed up (coolant at temp but oil not yet).

tpi 421 vette

If it's idling high and your iac counts are zero, it sounds like your throttle blade is adjusted open too far.

Ape0r

Throttle blades are not adjustable, and are fully closed (no visible gap when shut).

sailorsteve

With IAC counts at zero, you are on the right track to suspect an intake air leak. Since this condition has existed since you got the car, it wouldn't hurt to pull codes just for the halibut, there could be a stored code that wouldn't light the service light. There is supposed to be some sort of factory-applied "sealing compound" on the edges of the TB blades that they caution against disturbing by overzealous cleaning, maybe the PO did something like that. Also possible, but less likely, the ECM is at fault.

pcolt94

My personal success with cleaning a IAC motor has not been good with any of my vehicles over the past 20 years. They may work for a while, maybe good or not.

Now my approach to any kind of problem like this is to replace the IAC. Then at least you can move on confidently to another area if need be. However my experience is a high success rate in the past with resolving the problem.

I'm not saying you IAC is bad, I just like to know where I am.

**I see now that you changed the ICA, sorry I missed that before.

surfer93

TB, EGR and EGR tube can all leak around the gasket and did you take the TB off to clean and if so, did you replace the gasket?
Has the intake been taken off?
Did you check the intake bolts to see if any were loose?
Do you have a new PCV hose?
My PCV hose leaked, had to clamp mine.

When I get home, I will look at my FSM for more info and suggest(strongly) that you get one.

Ape0r

Car has no codes posted. Get those when I plug in my laptop. As I said above, have already replaced IAC.

Again as I said above, changed all three TB gaskets and intake gaskets along with the EGR gasket this weekend with no effect. Bolts were all torqued to spec and in the correct sequence, with steps up to the full torque.

No FSM, but I have an alldata subscription.

Have not replaced the pcv hose - it is on my list for this weekend. Carb sprayed everything thoroughly and gotten no effect. Plan to get after all the accessories with a vacuum pump after the new year and get them all sorted.

pcolt94

From the beginning in reading this post, the "zero" counts bothered me. The control and feedback is controlled by the ECM as does the information output via the serial bus.

Don’t discount that the ECM could be the problem. I hope it's not because anything else is easier to obtain.

Ape0r

Doubtful - checked this by placing car in diagnostic mode (which closes IAC), disconnecting IAC, and then cutting off diagnostic mode and then running car. Idle was identical to idle with IAC connected and showing 0 counts, meaning that the ECM is correct in thinking that the IAC is closed.

Verified IAC closure by doing the same procedure before disassembling throttle body this weekend. Filled IAC inlet port in coolant plate with water to check for leaks, and it held just fine.

IAC is operating correctly.

kimmer

Did you try adjusting the throttle cable?

Ape0r

Readjusted per FSM procedure after intake job. There is slight slack in the cable as it joins the throttle body, throttle body cam is fully bottomed out on setscrew (which is not adjustable), and throttle blades look like they are as closed as closed can be.

C409

........ Have you isolated the brake booster ? It would take a sizable air leak to affect idle and IAC that much ........

kimmer

Follow the cable from the tb till you get to a black plastic thingey. On the top it's kinda round and on the bottom it's flat. You can push in on the flat part and the thingey will move, this will change your idle rpms.

Ape0r

Cable has slack, meaning that it is NOT exerting any load on the throttle cam at the TB. TB is fully closed. This would only raise the idle from where it is, could not be used to lower it.

Re C409: I have isolated the brake booster.

pcolt94

I took some data from my Tech 1 (couple years ago) of every reading it can make recording 3 conditions. Key on, engine run open loop and closed loop. It's a Xl spread sheet with 150 points of data and would be happy to send it t you. However I thought I would just post my IAC counts as I recorded them.

Condition-----Key on------Engine run open loop------closed loop
IAC counts:---134---------------47---------------------28

This might be a compairson you can use in some way.

AVL94LT1383

Can you log/review pulse width, O2 readings, IAC counts and spark advance with your data software?
What is the programmed idle VS temp?
What is the min injector PW..... and host of other settings in the ecm???
Have any of the AFR open loop settings been changed.
What are the MAP readings?.
Fuel pressure?
Have any of the MAF closed loop settings been changed?
What is the IAC table counts for 0 speed.

IAC at zero seems odd, but if it is trying to lower Idle RPM it should also be removing timing and PW assuming the O2s are reading rich (Go Lean)....

Do you have tunercat and datamaster?

AVL94LT1383

Sorry, I think the 92 is a speed density car so MAF and programable bins might not be available. So what are your thougts about the MAP working properly?....

Ape0r

You are correct; no MAF.

Pretty sure all sensors are a-okay - here is a csv file containing 3500 data points from cold start to almost full temp (oil gets up to 180s). I drive around for the first two thirds or so, and the last third is just hot idling. Idle gets to ~800-825 towards the end, picks up to ~900-925 as the engine completes warmup (oil usually settles around 215F). Let me know what you think.

URL:
http://filebox.vt.edu/users/twg/run.csv

Link:
data

pcolt, would appreciate your data. Could you e-mail to me? E-mail is twg(at)vt.edu.

Ape0r

I had a theory tonight, maybe the (rather cruddy and cracked around one nipple) pcv hose was keeping the pcv valve from seeing any vacuum, thus allowing it to sit at full open and flow a lot of air through the engine and into the manifold.

The hose came apart when I was pulling it off, go figure. Will report back tomorrow after a car parts store run. Hope springs eternal.

pcolt94

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