(a little info before I get into this: 91 z28 5.7 TPI. It has an aftermarket TPI manifold, computer chip and the ENTIRE emmissions system has been gutted, all before I bought it.)

Ok so I've been through quite an ordeal with the 91 z28 I bought about a month ago. I brought it to a mechanic to get the ball joints fixed so I could pass inspection. He fixed the ball joints and also said I had to fix a rear differential leak and transmission leak. So I brought it back to him the next week and got that done. While he was fixing those things, he said he toned down the idle a bit (with some little adjustment screw). Well on the way home from the shop that day, the car overheated (I was just barely getting into the 260 range when I had to pull it over). The next morning, after starting it and driving it through my apartment complex to goto work, it was stuttering, as if was starving for gas. But it went away after driving it for 30-45 seconds. I'll get back to that later.

I brought it back that morning to fix the overheating problem. It turns out that the wire connected to the sensor that turns my second fan on was disconnected and burnt from the headers. So they fixed that. I mentioned the stuttering thing but they never really got around to checking that out. They were busy the next 2 days, and were on vacation they next week. During this time I figured out that the stuttering problem happens whenever I haven't driven the car for roughly 4 hours or more. And it takes longer to go away the longer it sits. Like when I leave work (car has been sitting for 4.5 hours since lunch) it would take a seconds before going away. But when I let it sit over night, it takes longer, like 30-45 seconds. Keep in mind that once the stuttering stops, it's smooth as butter. Now I don't know much about cars but if I had to guess I would say it's either something thats drying up and clogging something, and needs something pushed through it to clean it out, like a gas line OR theres moisture building up somewhere and it needs to heat up to dry it out.

Anyway, I bring it back to the mechanic today. Instead of telling me he fixed the problem, he has this hour long talk with me about what "path" I want to take with my car because of the condition it's in. He told me that he noticed that even though he had set the idle lower, it went back up. So he brought it down again, and it went back up again. He says it might have something to do with the computer, or the chip, or a number of other things. He says blah blah blah I don't know what was done to this by the guy you bought it from and that I should decide whether I want to gut the whole fuel injection system and go with a carburetor OR try to upgrade what I already have. Either way he wants me to buy a cheap busted up camaro for the parts. I'm thinking the whole time "wtf just fix it, it can't be that complicated". He did say that he replaced something to do with the fuel system, like a pump that was clogged or something. He said it brought the fuel pressure from 32 to 55 by fixing that. So I finally leave and at this point I'm just thinking I'll either take it somewhere else or ignore the stuttering, since it's only really a problem in the morning. Well now as I'm driving it home from the shop, there's a new problem. It's stuttering THE WHOLE TIME that I'm driving it. That was definitely not a problem when I brought it in. I mean now it's so bad it feels like the car is going to just die. Can you imagine my frustration at this point? That's twice I've brought it in and it's come back with problems I didn't have before.

So does anyone have a clue what could be happening? It really seems like whatever he did to the idle is what's causing problems. Maybe every time he adjusts the idle screw, the computer doesn't like it and somehow adjusts other things to compensate and bring it back to the idle it wants. Does that make any sense? Could the fact that there is more fuel pressure be doing anything? Like maybe even though the fuel pressure was bad before, 2 negatives made a positive and now that the fuel pressure is fixed it's throwing everything else out of whack? If I wanted to try to undo what he did with the idle screw, which way should I turn it to bring it back up? Please help, I don't want to drive my tercel all summer. Thanks

 

Find a different mechanic. The idle speed is completely controlled by the ECM and IAC stepper motor on every fuel injected GM vehicle produced since 1984. I know that this nearly 25-year old technology may be "new" to your mechanic, but you don't need him/her learning at your expense.

Unfortunately, adjusting the throttle position stop screw has now probably altered the TPS voltage at start-up. Even though the 1990-later TPI systems should learn the startup TPS position,there ARE limits for a minimum TPS level. You need to get a tachometer and adjust the throttle minimum air position. You can search the board archives for instruction for that procedure.

The fuel pressure is NOT "fixed" now - It is broken. The fuel pressure should NOT be 55 PSIG. The specification for a stock TPI is 37 PSIG ±3. The 32 pounds it had before it was "repaired" was a lot closer to the specification than it is now. Peak fuel pressure (engine not running or WOT) should be 44 PSIG ±3. The current 55 pounds of fuel pressure is too high, and may be damaging your fuel pump ($$$). It is almost certainly making the mixture too rich, and can also damage the injectors. That's just another reason to find a different mechanic. Reading a manual or specifications is a rudimentary step of the repair process. Apparently, that's not necessary in that particular garage, so drive away soon.

And if you go back, you can expect more of the same. The suggestion to "gut the whole fuel injection system and go with a carburetor" is wrong on many levels:

• If you have an emissions requirement in your area, the vehicle will NEVER qualify;
• In terms of reliability and daily driving satisfaction, a carburetor would be a quantum leap backward;
• Your fuel mileage will not improve, and will most likely decrease, costing you even more money.

That comment reveals the mentality of the shop, and is the only reason you need to never return, let alone all the other reasons you already have.

 

Thanks for the response. Great info. I did some reading to help me understand the problem better. First thing I noticed in this article: https://www.thirdgen.org/tpimod1 was that 55 PSI doesn't seem that far off from what is suggested (50). My camaro definately isn't stock. Do you think that a fuel pressure around 50 would make sense seeing as I have an aftermarket manifold?

I also found this article: https://www.thirdgen.org/tpimod2 . It says to adjust the idle with that screw. But I thought you said thats bad news? It also says to set it to 450 rpm, which, as I understood it, will somehow make it go to 600 when I shut the car off and start it up again. Now that 600 is for "unmodified cars" and mine is quite modified. How do I find out where my idle should be? When it was running fine, it was around 1000 when I was stopped at a red light or something. I really think what I need to do is somehow put the screw back to where it was when the car was running smoothly. No idea how to figure that out though.

 

bump

Would it be a bad idea to just go outside, start the engine, and turn the IAC screw a full turn clockwise?

 

Well, I went ahead and did it anyway. I turned the screw a little bit and started the engine and drove it around for a bit. It seems a little better but I'm still noticing the stutter. I don't really understand how adjusting that screw will help though. I mean, from just looking at it, it seems like all the screw really does is mechanically give the throttle a minimum position. I don't see how that would affect the car once I'm stepping on the gas.

Also, if this helps, I'm getting a bit of RPM pulse whenever I'm stopped with my foot on the brake. It pulses from like 1000 down to 970 (rough guess) in like 1 or 1.5 second intervals over and over. When I put the car in park, it shoots up to 1400 or so.

 

Adjusting the throttle position alters the TPS sensor reading, and can affect how the engien starts and operates in decel, since the IAC is positioned by counts on ignition power-up.

Follow the instructions and set it to 500-550 RPM in DRIVE with the IAC fully closed and disconnected.

 

I wasn't sure where the IAC connector was and I didn't want to go pulling wires randomly. In the image below, I believe it's the connector right below the TPS. Is that correct? If so, whats the best way of taking it off. I'd rather not break it. Thanks



Edit: I've done some more reading. I found this:

"At idle, the TPS should be read .54 volts (factory specification) unless it has been set to a different value inside the prom."

If I have an aftermarket chip, how can I find out what my TPS voltage should be?

 

from the pic you have your high idle is coming from your throttle body ,looking at the pic the top hose on the throttle body is supposed to be hooked to your vacuum canister ,or it needs to be plugged .its giving you a vacuum leak .later jimmy

 

1fastam: The top hose is plugged, it's just hard to tell in that pic,

Ok so I found the IAC connector and tried it out, but with no luck. It may even be worse now. My order of operations:

- A-B jumper
- Turned key on
- Waited 30 seconds
- Disconnected IAC connector
- E-brake
- Put car in drive
- Loosened the IAC screw, but the RPM's stayed @ 1000 no matter how I changed it
- Realized I should probably be doing this at normal operating temp
- Shut the car off
- Reconnected IAC connector
- Pulled the jumper
- Drove the car around for a bit
- Brought it back and shut it off
- Tried to do the whole deal again but the car wouldn't start w/o stepping on the gas (presumably because the IAC was so low from what I did the first time that it wasn't getting enough gas)
- Tightened the screw enough that I could start the car without stepping on the gas
- Shut the car off
- A-B jumper
- Turned key on
- Waited 30 seconds
- Disconnected IAC connector
- E-brake
- Put car in drive
- Surprisingly, the RPM's were at 550 like you suggested, so I said "sweet" and went with it.
- Shut the car off
- Reconnected IAC connector
- Pulled the jumper
- Now when I started to drive it, I had thought it was fixed. It was pretty smooth out of my apartment complex, through a red light (idling at around 800 while stopped), and over the highway. But it didn't last. I got in some traffic on a hill and the RPMs were surging between 600 and 900 or so. When I got out of the traffic, I was experiencing that stuttering again. I turned around in a parking lot and stalled out with the steering wheel all the way to the left (is there any reason the car would feel like it's more likely to stall out while doing slow tight turns?). Basically it's running like crap.

I tried checking my TPS voltage, but I must be doing it wrong. I turned the ignition on, disconnected the plug that goes to the TPS, then touched the negative on my DMM to the contact that the black wire goes into (assuming that's the negative), but there were two other contacts (white and blue wires on the plug). I tried putting the positive on my DMM into each of them but neither gave me a reading. Do I need to jump those other two contacts together?

I also have a new problem. While driving I noticed a high pitched constant noise that seemed to change as my RPM's changed. It sounded kinda like wind blowing through something and making that "ghost" noise. But it wasn't wind since it persisted with the car stopped. It seemed to be coming from the back of the car. I hear a quick "weeeeee-ooooooooooo" from the back of the car when I turn the ignition on (not engine on) now and kinda hear it again whenever I stop the engine. At first I thought it was my electronic hatch motor, but now I'm thinking it's my fuel pump.

 

Pushing a camaro across a busy 3 lane intersection is fun. 3 people were actually nice enough to jump out and help me without being asked! I just took it out to see if I was still having the same problems after letting it sit overnight, and it died on me at a red light. It kinda went out slowly, acting like it was gonna die, then coming back. Finally it just gave out. I tried starting again a few times and it kinda caught, but then went right out. Using the info in my previous posts, would you think the fuel pump is dead?

 

Okay, I'm going to try to explain this very simply. Please follow the following procedures exactly for setting everything back to normal.

1. Set your base idle speed.
A. jumper the A - B connectors just like you did before
B. turn the key to the on position (do not start the car with the terminals jumpered, this could possibly damage the ECM)
C. wait 30 seconds and then disconnect the IAC. It is a 4 prong connector just below the TPS sensor.
D. remove the jumper
E. start the car
F. using the screw on the throttle body (IAC adjustment screw), turn the RPM down to 450-550, whichever one allows you to keep the screw in contact with the throttle....if you back off of it too far, it's not right) There is a specific number it is supposed to be, and there is a specific gauge you're supposed to use, but just use the car one, and just make sure it is 450-550 and you'll be fine.

Now that your base idle speed is set, turn off the car, plug the IAC back in, and keep the car OFF, but turn the key to the on position.

2. Setting your TPS
A. you must leave your TPS connected during this process. Get a voltmeter with long skinny leads, or make long skinny leads to go over its leads, that you can poke through the connector, so it makes contact while the tps is still connected.
B. looking at the TPS from the side with the screws, loosen the two screws just a little bit each, enough so the tps can move around just a bit
C. you should notice the voltage changing during the above process. if you're still looking at the TPS from the side, if you rotate it counter clockwise, the voltage will jump up, if you rotate it clockwise, it will go down. get it to about 0.54 and then tighten the screws
NOTE: as you tighten the screws you will notice the voltage continue to jump around. make as much effort as possible to maintain 0.54 vdc until the screws are tightened all the way. when i did this, i settled for 0.55 vdc, after trying to tighten it several times. you should be able to go 0.54 +/- 0.075 vdc.

3. Unplug your battery (or ECM if you don't want to lose stereo settings)...wait a couple minutes to make sure the ECM resets.....you want the ECM to basically throw away all that garbage sensor reading you were teaching it with, and let it relearn with the new, correct readings.

4. for your fuel pressure issue.....there is a schrader valve on the passenger side fuel rail....get a gauge for that and check to see what it is at idle and running, and see what it does when it starts to sputter. if it goes up/down while it is sputtering, it is a fuel issue. if it doesn't, its a vacuum issue. and im willing to bet its vacuum, personally, because someone gutted all of the emissions stuff and probably forgot to plug something off, or connect to hoses or whatever the hell happens in the emissions system....but check your fuel pressure first



oh and last step is....punch your mechanic in the throat