Car: 89 5.0 HO Firebird Formula with 305TPI

My brothers problem is when he starts the car up it jumps to 1500 rpms and holds there and nothing will get it down unless he turns the idle screw. Once done and brought down to like a resonable 800 rpms or so the car seems to develop a hiccup or rough idle condition and eventually forces you to goose it to keep it running smoothly for a couple of minutes before returning to the rough idle condition.

He has changed his plugs, wires, cap, rotor. I told his next to try to run the car and as soon as it jumps to the 1500 to shut it off and take off the throttle body, clean it good and remove the IAC and check to see where the pintels position is. Then clean is really good as well and try reinstalling it and see if that helps. If not this to try changing out the TPS sensor. (cause we have an extra off my 1990 vette and I think it would work)

Is this the direction you guys would say to go as well?
Jeff

 

Can you scan it to look at IAC counts and TPS position ??

 

i haven't learned yet how to hookup a computer and run a program. In the process of that one, I have the cable and laptop but haven't figured out yet how to run the damn program....frustrating indeed. I can hook my multi meter up to the tps for voltage though. What do you think I should be seeing? around .5volts? and if it is jumping around would that mean bad TPS?

 

Id be inclined to , a bad TPS could do this....

 

Idle can be tricky as there is little margin for error. If anything is a little bit off you get a rough galloping idle and possible stalling.

You need proper tps voltage, proper ignition timing and a good open loop calibration and a properly functioning IAC. If there are any mods, you probably need a custom tune, or at least a heated O2 sensor to get into closed loop easier, which can provide some mixture compensation.

Proper tps voltage ensures the tps position isn't adding counts to the IAC position in throttle follower mode. This may be your problem, or you have huge air leaks or the throttle stop is screwed in way too far, which could also be raising the tps voltage out of the idle range.

Get some scan data to see what's really going on. TTS Datamaster demo is free for 20 recordings and easy to use. TunerproRT is nice but may be harder to get started with.

The maf calibration can cause problems since there is a kick up in the table #1 at the lowest flow/voltages position. This causes the flow to be over estimated and the load artificially increase and causes a very rich condition as the car is already dying.

Any mods we should know about?

 

As per corvette. TPS voltage should be key on .5v and closed throttle and 4-5v WOT. Vacuum leaks could account for the high idle. Does it idle high in open loop as well as closed loop?

 

Ok car when first started up mode (open loop I believe) is at idle at 1500 rpms and is at .7 volts tps. WOT I have to check again but think it was somewhere around 5. Once warmed up it kicks down to 800 or so but has a surging chuggle to it.

I gotta try and get the laptop going

 

Try adjusting the TPS down to the number I gave and see what changes. (easy to do)

 

0.7 volts bingo!

That will add around 20 counts (maybe more) to the normal IAC position, depending upon the gain factor used. This would definately cause a high idle, since according to the tps voltage this is part throttle, not idle any more.

 

Ok so I changed out the TPS sensor with one from my 1990 the only difference between 1989 and 1990 is one is adjustable and the other is not...strange and oddly when I took his out it was reading .71 volts at idle which seems to be off (I thought it should be around .5 like you guys said) BUT. after putting my non-adjustable one on, it again read the same exact voltage of .71? so hmmmmm I dunno. PLUS Iam now wondering if it is or istn't an IAC related problem cause the car idles at 1500 when first started up and although the idle is screaming the car feels smooth. BUT once the car warms up and the idle kicks down by itself to 800 rpms or so it then develops its missing, chuggling or loping type of feel. What do you guys think? Hopefully this additional information might shed some light. Iam going to try to get the laptop working today.

Jeff

 

First, it sounds like the whole idle, minimum air, IAC, set up is way out of whack.There is no idle screw. The idle is supposed to be controlled by the ECM manipulating the IAC. If backing off on the throttle stop (minimum air adjustment) screw, lowers the idle speed, the minimum air adjustment is all screwed up.

In this last post, you've change your story from "nothing will get it down unless he turns the idle screw", to "once the car warms up and the idle kicks down by itself to 800 rpms"Which is it? Also, if the first report was correct, the IAC may have been adjusted property to read .7 volts at a 1500 rpm idle, because in the first post, the high idle was the result of the throttle plates being cranked open. If the throttle stop screw were backed off to a proper minimum air adjustment, the IAC voltage would likely have been correct.

There are too many conflicting symptoms, here. I think the best thing to do is start from scratch. Clean the IAC passages in the TB, and start over with the minimum air and IAC adjustments. Then if things aren't right, come back to us. At least we will know where we're starting from.

RACE ON!!!

 

Yeh I see what you mean CFI Iam kinda conflicting...now that I re-read it....I have to do a couple more tests and have already cleaned out and reinstalled the IAC and reset the idle air screw to where it should be.

Basically here is where iam at right now.

Reset the air screw on the throttle body to be only a few turns in. Before it was almost all the way screwed out. This makes the plates almost completely closed. The result is at first starting it up it goes to about 1100 rpms now and then back down to 800 or so.

Once it dies down is when it starts to stumble and skip.
I haven't yet got the laptop up and running so thats the next step. Been too busy with him trying to get the brakes working.

Ill get back when i have more computer data on what the hell is going on with the car.

jeff
thanks for the help so far

 

OK, after all is said and done, think it might be related to EGR functions.

We changed out the TPS and the IAC and played with them to get the minimum air correct.

We also changed the whole distributor so its brand new.

Car now runs perfectly fine now when first started up and while it is cold.
Once it warms up, the problem begins.

Stumble, rought idle, skipping feeling. No check engine light, so no codes.

I tried scanning the car, but for some reason my code scanner and computer won't connect to his 89 ECM so thats out.

What you guys think?
Thanks
jeff

 

Hey ExtremeVette- just so happens that at this exact time, I am also having the exact same problem with my 2nd car- '91 T/A 305 TPI and it's driving me crazy. I've replaced the exact same parts that you have and also replaced the EGR since I was throwing a code for it. Does your burn smell rich? Also, have you checked the timing. I'll be on vaca next week and that is one of my projects- to find out what is causing this problem that never sems to get better no matter what I do to it. I've datalogged the car and I think I"m getting closer to finding out what's causing it. If I come across any different info that could help I'll let you know. If you'd like a source that show you how many people have had the same problem and what they've done, go to www.thirdgen.org and read the forums after doing a search. Their site is just like ours here.
Also, just a thought, check all your vacume line for leaks and or cracks or decay. The smallest leak will definitely effect a stumbling idle condition.

 

Gas comes in 2 liters as well. Pull my finger and I'll give ya a couple.

Charlie

 

It sounds to me like its leaning out during/after warm up as the startup AF ratio enrichment backs off. Can you increase the fuel pressure?

If you don't have an AFPR, try pulling (and plugging) the vacuum line that goes to the regulator for a simple test.

 

See my topic
http://forums.corvetteforum.com/show....php?t=1414221

 

Still no luck with car. Re-Ran it for codes. Nothing.
Disconnecting MAF thinking that might have been it. Same problem.

Car runs perfectly smooth now when it is in open loop. As soon as she goes into closed loop and drops the idle and begins running like poop. KEEP IN MIND CAR FAILED EMISSIONS CAUSE OF NOX.

same symptoms:
rough idle at 650-700rpms
feels like a skip and stays throughout entire rpm band even as you rev it.


need more help in diagnosing this.
Thanks

 

I still think you're running lean.

NOx peaks with a lean mixture (around 16:1 AFR). Rough running is also a lean symptom as the lean limit (lean misfire) is approached.

As you get really lean (18:1) HCs will begin to increase due to the unburned fuel from the lean misfire condition, but NOx should be coming down at this point. I guess your running somewhere in between the NOx peak and lean misfire limit.

Again, I would suggest raising the fuel pressure, unless you have some other means of enrichening the mixture (chip tuning).

 

Hey Jeff. I found the problem with my T/A. I found that the IAC was not seated completely in the throttle body. I removed it, cleaned it and the orafic it fits into, and reinstalled it nice and tight. Idle became steady immeidiately. Just thought I'd pass that info along in case your still having the same problem.