Ok so I know there are several of us having similar problems with out C4's right now with hard starting, stalling, and rough/erratic idle. IDK what all they've tested but I compiled a list of all the possible causes (out of the FSM of course) and have been narrowing down the list. Heres my list thus far... (if its bold it has not been fully tested yet)

Symptoms:

Hard Start, Stalling, Erratic Idle

Possible Causes:

-TPS (no binding, less than .700 volts)
-Throttle Linkage for sticking/binding
-Coolant Sensor (high voltage)
-Intake Manifold Leak
-Fuel Pressure
-Water Contaminated Fuel
-Fuel pressure regulator
-PCV valve
-EGR (none on startup)
-IAC
-Injector Fuses (check both)
-Ignition Timing
-Alternator output (no less than 9, not more than 16)
-Ignition system
-worn shaft
-correct output ST-125
-bare and/or shorted wires
-pickup coil
-coil pack
-moisture in cap
rotor
wires
-cold start valve
-fuel pump
-fouled plugs
-MAF sensor
-Air leaks
vacuum leaks
-O2 sensor
-Defective ECM
-Cylinder Head bolts loose

I have not 100% ruled out the timing yet because I cant keep it running long enough to use the timing light. I'm not sure how to test the O2 sensor though or the alternator charge. I dont want to get fried sticking my volt meter on there! Any advice on the proper way of doing this?

 

You've got to be kidding me, no one else has had this kind of problem before? I just got home expecting some of you experts out there to give me some advice, theres no way i'm the only one thats had to go through this much diagnostic crap.

 

Have you checked the injectors? That would be the first place I'd look! (been there...done that) It's simple to do..squeeze the spring clip that holds the wire connector to the top of the injector to pull it off. Put a digital voltmeter on to the injector contacts..and take a resistance reading. They should be 16 ohms (I forgot what the tolerance is...but I think + or - 3 ohms) If you get a reading a lot under 16 ohms, I would bet you dollars to doughnuts you have an injector that works sometimes, and doesn't work other times. If you have 3 that don't operate properly at the same time, your car will runn like crap!
Let us know how you do, and what you find out.

 

in this order :dirty IAC / bad IAC , weak/bad/inop fuel injector / fuel pump / vacuum problem / distributor
(and all parts inside distributor)
All this ASSUMES you have checked and or replaced wires/plugs/cap coil/rotor/

 

The IAC is less than a year old, I checked it and its clean. I also tested it according to the FSM using a multimeter. I have a new fuel pump and filter. The distributor and related components are all good or have been replaced. I have not ohmed my injectors and dont feel the need to as my fuel pressure is just fine, no leaking and they're all working.

 

what is your iac numbers..is it working - year old or not, how do you know your injectors are working, and if working spraying mist not dumping fuel. How did you test module, pick up coil?

 

The IAC does work, I can hear it moving in and out with the idle and it ohmed at about 54 ohms which is above the required 20. I did a similar check with the distributor components.

 

4ever21: Have you considered the option of finding someone to scan your car's ECM? This process identifies the "area" of your problem. You concentrate on the "area" and the problem is much easier to locate. Find a local Corvette Club and ask who is good at using a scan tool.

 

I've got a scan tool, but its not going to work if I cant keep the car running long enough to collect information. I"m not the best at it mind you, but idk of anybody else in my area thats good at it, except maybe a dealership. I dont have the money right now to take it to a mechanic or I would. I only know of one mechanic in my are who's good at diagnosing TPI, but i've dealt with him before and he's the biggest d*** ever and a rip off. I swear i've never seen a car so finicky and touchy to even the slightest touches.

I do appreciate everyone's help so please dont think i'm trying to be a jerk or anything. Its June now and I have hardly been able to drive it at all this year due to all the problems its been having, so i'm sure some of you know how I feel right about now.

 

Plugged up catalytic converter? (not sure if that was one of the things you already checked or not).

 

Your O2 won't matter at start up. There are a few ways to test the O2 but when you say the car does not run, what does it do? Fire up for a moment then stall? Does it run super rough? Does the idle hunt and drop so low the car stalls? Will the car run if you give it some gas? Have you checked fuel pressure? It could something simple like the pump. You can run the pump with a 12v jumper to G on the ALDL for my cars year.

Alternator won't matter for rough start. Check to make sure your battery is charged to look into troubles.

Head bolts loose would be a whole lot of signs once the gasket let go.

ECM, get one and try it. Return it if there is no change. Does your display codes? They are easy to change.

Injector fuses? There is one in the fuse box? Our cars are different years so I am not sure what you mean with that. I keep an old light bulb from the dash and plug it into the injector clip, if it lights up the signal is good.

 

-TPS (no binding, less than .700 volts)

The 85 may be different, but I assume it is like the 88 in that the TPS should be 0.54v =/- 0.08v. At .700v you would be too high.

The IAC needs to do more than operate to be working correctly. It needs to be working in the proper range. The FSM has a section on minimum idle speed ajustment. Since you report that you have a new IAC, you need to set the minimum idle speed adjustment based on the specs in the FSM, if you haven't done so already. In my 88 FSM it is covered in section 6E-C2-12.

A laptop based scan tool like the EASE OBD1 scan tool (and some others) can be activated prior to starting the car. You can then watch/record the values reported by the ECM during initial start and run attempt. Sometimes that helps define the area where the problem exists. It will also record intermittant DTCs (Diagnostic Trouble Codes) that sometimes don't show up as hard codes when using the paperclip method. If you get no info or garbage info from the ECM during start-up, you have reason to suspect the ECM. If your scan tool will do this, I suggest you use it to try to see if anything is reported when you attempt to start the engine.

Good fuel pressure only tells you that the injectors are not leaking excessively. The fuel pump, when in good condition can compensate for one or two leaking injectors without noticable fuel pressure loss. Also, if no injector was working properly, (i.e. way too lean), you would maintain proper fuel pressure as well. You don't loose pressure when the injector doesn't open, or open properly.

The 85 is a batch fire TPI. The driver side or passenger side fire all 4 on that side at the same time. One shorted injector will stop the entire left or right bank of injectors from firing. It will have no noticable effect on fuel pressure if they don't fire and the other side does. To say that pressure is good, therefore the injectors are good is very misleading. An injector ohm test is critical here and it is easy to do.

 

sometimes whatever the cost mechanics / technicians are worth the money, people cant buy tools, go to school, and give their knowledge away for free, I am sure there are a lot of places that can diagnose your problem.

 

The TPS tested in at around .57 volts and would steadily increase in voltage as I increased the throttle.

The ECM is still setting trouble codes if I unplug something and it will give code 12.

I suppose it could be possible for the head bolts to be loose. When I put the heads on I tightened them down to correct specs (65 ft. lbs top bolts, 60 ft. lbs. bottom). I checked a few of the bolts on one side and they were still at the correct torque. They are aluminum heads. I dont see anything leaking at all, so i'm not sure if its a head gasket failure.

Right now when I start the car it will idle for a second or two like the rpms will go up and then down, then stall. I can smell unburnt fuel so it seems unlikely that its NOT getting fuel. It will stay running if I give it gas. When I do this it sounds really choppy like its missing or something which is why I suspected something was wrong with the distributor. Its not backfiring though or knocking like its lean.

At this point, if I knew a reliable mechanic that knows how to work on TPI engines then I would take it to them. If anyone knows of such a mechanic in my area, please share

 

Has it run since you put on the heads or replaced the distributor? Easy to get the distributor off by a tooth (well at least for me).

 

4ever21: A couple of suggestions for you:
1. Check the firing order of the plug wires (1-8-4-3-6-5-7-2) and this is clockwise while looking down at the distributor. The #1 wire should be about 1:30 occlock with 12:00 being the front of the distributor.
2. Check the timing to be sure it is 6 deg BTDC ....... with the EST connection open. If it will run for a few seconds......you can check the timing. As suggested by SunCr, it is very easy to be one tooth off and this will cause problems such as you have.
3. Check ever vac fitting on the plunum and surrounding area to be sure you have not missed a fitting.
Hope this is helpful.

 

Yes after I got the heads on and everything set up it ran great for 2 full weeks and then I had a leak in the fuel rail. The o-ring at the cold start valve had broken, so I replaced it and it ran great for like another day or two. Then it started doing what it is now.

I've checked all the vacuum lines, they're nice and snug. The firing order is correct, and i've got the timing in a place where it runs the best. I used the old method of rotating the cap until it starts to run and I have it in a spot where it runs the best so I would assume its around 6* BTDC but more than likely not dead on.

 

Post your scan data. Key on engine off. All of it. The whole list. Next, use some carb cleaner and spray it into a vacuum hose while its trying to idle. If it picks up its either not enough fuel, or something telling it to not add enough fuel. You need to work a few hours of overtime to make enough money to take it to a good diag guy. You been screwin with the thing for GOD knows how long?

 

I'm sure you did disconnect before setting the timing didn't you?

 

Since it ran fine for 2 weeks prior to the cold start valve fix, I'd look very closely at that specific area and the components that you worked with while repairing the cold start valve (CSV). Is it possible that an injector connector near the CSV was not pushed down onto the injector completely and has come loose? Are there any wires or hoses in that area that might have been incorrectly re-attached or are now loose? Could you have shorted one of the wires of the injector harness or nearby harnesses? Have you double checked that none of the spark plug wires are loose at the distributor or at the spark plug?