'94 bucking severely and hard starting when warm
 

Hi Guys,

Long-time Vette guy here. I am on #9, but this is my first project car. All of my previous Vette’s were in good condition, including a restored ’71 and a ’15 Z51 that I also own. I am mechanically inclined, but it’s been quite a while since turning wrenches on a car. I have surfed this Forum and other sites quite a bit about my symptoms, and I have some ideas. That said, I could really use your advice on the steps that I should take to test and rule various things out. I am dreading a diagnosis that I am experiencing the dreaded Opti-Spark failure, but I want to be as confident as possible that it’s the problem before I undertake that nasty job. Here are the facts:

+ It’s a 1994. Car has 72,000 miles on it. It’s a 6-speed.
+ When cold, starts easily and runs smoothly. Once warm, it “misses” and bucks severely when at low RPM, under load.
+ I think I have a stuck thermostat as the car continues to increase in temp to the point where I need to shut it down to cool it off.
+ Once warm, the car is very hard starting. Quite a bit of cranking to get it re-started.
+ I jumped pins #4 and #12 and the car displayed these codes on the digital display:

H15 Coolant sensor low temp indicated
H32 EGR Failure
H34 Map sensor signal low
H36 Ignition faulty high-resolution signal

Is it possible that the MAP sensor and EGR “failures” could be the cause of my issues? If this is an Opti-Spark issue could it in any way be related to the EGR and MAP issues?

I welcome any and all advice on how to troubleshoot my challenges!

Thanks,

Jim B

Your original complaint is classic opti with the low RPM and bucking……however.
(H36 may indicate a opti high resolution pulse failure)

I would clear the codes, drive the car and re-check the codes.

What is the digital temperature on the speedo that you think is too high. That sensor (in the water pump) that drives the temperature on the speedo display also inputs to the PCM and is a real important as it is a main driver for the A/F mixture. Is the car overheating or do you just don't like the temperature? What temperature do you shut down?

I am not addressing the rest till we know if the codes are valid.

Thanks, Paul. I'll reset the codes tomorrow and will drive it and re-pull the codes. I don't think the car has officially over-heated. I just saw it getting up in the 260 range (on the analog gauge). I'll put the temp on the DIC and will see what that number looks like. At what temp is it actually overheating and I should shut it down for sure?

FYI, I installed a new radiator a week or so ago. I just got this car a couple of weeks ago and it had a leaky radiator. The seller had a just purchased a new radiator but had not put it in. I just bought a new 180 degree thermostat but have not put it in. Let me know if you think I should go ahead and do that.

If no AC the left fans comes on at 219* and right side fan comes on at 239* as what's in the FSM.
When the AC is on the left fan well run. Here in FL, I run the AC 12 months a year and it's hot about 9 or 10 of them. If it's 75 or less it should run about 200 (or less) but if you stop the temp will rise till you get moving. If 85 or more it will run 210 but if you stop it might go to 220. I am being real general just so you get an idea. All temperature readings are digital as that what is used for the accurate reading. The analog gauge is for reference only as some are not very accurate. I never get near 260 way to hot.

I have also installed an auxiliary switch where I can turn on the right side fan as I desire if I want to keep it temp from rising to high. Personally I like to stay under 220 all the time and when it's 100* here and stopped, you really need additional air flow to keep it from getting to hot which is my personal choice as I don’t care what the engineering says it can be.

I thought the block sensor fed the computer and the water pump sensor fed the dash?

NO, the sensor in the head (block) feeds the analog (dash) gauge only.
And that's probably for 92-96, (if not more ?).

If you get stuck working on this let me know. I have a GM Tech 2 clone and Snap-On MT 2500.

This is really good stuff, Paul. Thank you. I'll report back later tonight or tomorrow.

Plugs
 

Here were my issues:

1994 ZF6 with 89K miles.

1. Severe bucking at very low RPM in gear, WOT, heavy loads, high RPM's.
2. Cold starts were perfect every time. Warm/Hot starts were very rough and the car died often. I just couldn't idle.

I decided to order plugs, wires, fuel filter, Ignition coil, and Ignition Coil Module to get started. I ordered all OEM AC Delco parts and TPIS 8 MM wires.

The plugs, coil, and ICM showed up and the wires are still on the way.

1. I replaced the plugs with AC Delco DP and I probably solved all of my issues. The car pulled hard to the redline and could handle a very heavy load at a low RMP and not buck. Zero bucking under any test. I did not test the warm/hot start after just doing the plugs.

2. A couple days later I replaced the coil and ICM. I used the correct heat sink paste and took my time. The car now runs flawlessly. I also did several warm/hot start tests after shutting it down and after letting it sit for a period of time (still hot then later warm)... ZERO issues. I am very happy.

I never thought the plugs could have been the issue. I still think the ICM contributed to the warm start trouble but I am not sure.

Before we condemn the PCM……..

Does the temperature indicate the ambient temp before the engine is started ? Think you said no.
With the new sensor in the pump is it the same problem with the old sensor ?
Could the new sensor be defective?

Does the left side fan come on when you turn on the A/C ?

Do you still have the sensor that you removed ? If you do, make a resistance measurement on it. You can heat it and cool it and see if the resistance changes. You can use hot water or a heat shrink gun. Do you have a FSM or a chart for the resistance vs. temperature. A temperature of 142* would indicate a resistance of approximately 670 ohms. (77* = 2796 ohms).

Look for any wiring problems as well as any corrosion inside the connector. Hard to say why the temperature does not change.

With the key ON and engine off remove the connector and measure each pin. One should have 4.5- 5.0 volts on it. Then key OFF. Then measure the resistance of the OTHER pin only to ground. It should measure (very near) 0.0 ohms.

If the ECT sensor does not change value and tells the PCM the engine temperature, then the fuel A/F is set wrong as other engine operating parameters. As well as telling the fans to come on at 219 and 239 (with no A/C). That's why you are overheating with no airflow.

Thanks, Paul,

- Digital temp shows 142 no matter what, even before the engine is starts. Never changes from 142.
- Same problem as with old sensor. I tested the old sensor. it measures 1,210 ohms in 118 degree water.
- I measured the connector with the key on. One pin has 4.99v. Then, with the key OFF, the other pin measures 5,500 ohms. That same pin measures 1,700 ohms with the key ON. I don't see any corrosion in the connector.
- The AC has no freon in it (another project), so no, the left side fan does not turn on.

You said that the resistance measurement on the connector pin should be near zero. It's at 5,500 ohms with the key OFF. What does that point to? Wiring? PCM?

The other wire without the voltage should measure 0 because it's the ground return for the sensor. It is grounded thru the PCM pin B6. That point B6 is also the return for multiple sensors like TPS, oil temperature and transmission and more.

You need to chase this ground problem. Either it's high resistance for just the ECT, or all the sensors have the same problem and none will work right. The ECT might be picking up its ground thru all the other grounded items and that’s why the problem is happening.

So check out the wiring back to the PCM. It's possible the "B" connector may needed to be reseated and or cleaned. Checking B6 to ground (black wire) on the PCM may not be an easy task to get in there and figure out which one it is.

Before disconnecting any cables, remove the positive battery cable from the battery. Keep the negative one on so you can make a measurement (that should work). Do not connect the car battery until ALL PCM cables are reconnected.

EDIT:
Decided to check out your symptom and check out the ground return on my car.
I first unplugged the connector and turned the key on. I measured 5 volts on one of the connectors and my digital temperature on my speedo was 142. That would conform your open ground.

Then key off and measured the ground terminal which I got basically 0 ohms.

So you need to find out if it's just the CHT that has an open ground or all of them. So you pick another sensor and measure the ground on that one since all the grounds are tied together. The easiest is the TPS (throttle position sensor). Measure the black wire to ground, if reads 0 ohms, the PCM and connector are OK. But if you read the 5500 ohms, then you have to go and find out if the PCM is bad or it's a bad connection/wiring/splice problem

OK, I'm an idiot. I was doing the first test by myself and I screwed-up the test (probably when I had to go back and turn the key off). Anyway, I have performed the test on both pins multiple times (with someone else turning the key on (for the voltage test) and off for the resistance test. All of the tests shows 5.0 volts to one pin and 0 ohms to the second pin. I also checked the IAT and the TPS. Same results.

Thank you for checking on your vehicle. Sounds like 142 is a "default" of sorts. Anyway, do the results that I had above confirm that the wiring between the ECT and the PCM is good? If so, what's next? I'm not sure I'm qualified to mess with the PCM itself. :-)

Sorry for the confusion on the first test!

EDIT: I pulled the codes and got a C12 and H15.

C12 means there are no codes stored in module 1 of the CCM.

The H15 is a code in module 4 for the CHT. The H means it's Historic but it seems to be directly related to the CHT problem.

If what you say is true and the ground is 0 ohms, then that seems to be OK. The 5 volts to the CHT is provided by the PCM and you have measured that unloaded. When connected the 5 volts is loaded and should drop down to about 1.5 to 2.5 volts depending on engine temperature. You have to connect and back probe this connection to get the reading. You can insert a pin in the rear of the connector. Then hang clip lead on it and connect your meter.

If you measure 0 volts on the circuit, that would indicate high resistance connection (or bad PCM). In essence, the PCM is not seeing the CHT as to why the code is set and the digital temperature is wrong. There could be a high resistance connection in the 5 volt line (pin or connector connection) causing this problem.

The PCM voltage comes from pin C25 to the pin on the CHT where you measured the 5 volts. If you removed the "C" connector on the PCM and measured that wire, it should read 0 ohm on the wire. Don’t forget to disconnect battery.

The PCMs are a really good unit unlike the ECM and hardly remember people having a real problem with them.

Thanks, Paul. I'll do this test next.
Where can I find a diagram of the pin numbers for each colored PCM plug? I haven't pulled one yet. Are they numbered on the plug? Even if they are numbered, I'd love to know what each pin does (like you describe for B6 and C25).

EDIT: I found this source for the PCM maps. https://www.thirdgen.org/forums/engi...ticky-lt1.html
There are images posted of each plug, with descriptions for each pin. If you are aware of a better source, please let me know!

Those are the same pictures that are in my FSM.

You seem to me able to work on the car and have capabilities. It would be wise for you to invest in a Helms FSM. Other than troubleshooting aids, it shows you how things are connected, signal flow, principals of operation plus much more that are specific to your year of corvette.

Many times even for a beginner it will give you information of what to look at, where to start and what components might be involved.

Figuring out what pin is what wire can be a challenge. Be careful if you are going to remove the PCM connectors as they are old and the lock clips can break easily.

So, dig this, my problem with the ECT turned-out to be with the 2-pin plug itself. Remember when I went from getting bad readings to getting good readings? Well, that was because the plug was bad. I think it had been back-tested one too many times by the prior owner. Replaced the plug and the car runs like a dream when cold AND hot! Thanks for your help along the way. It was a learning experience for sure. I am SO happy that it wasn't the dreaded Opti!

Glad you got it figured out. :thumbs:

The CHT is such an important unit to work correctly to make the PCM do the right things. A high resistance in the CHT will usually make the engine run rich and many other things will be set wrong and will run like crap.