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'94 bucking severely and hard starting when warm

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Old 04-15-2018, 07:51 PM   #1  
JBOSS
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Default '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
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Old 04-15-2018, 08:55 PM   #2  
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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.
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Old 04-15-2018, 09:17 PM   #3  
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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.
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Old 04-15-2018, 10:06 PM   #4  
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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.

Last edited by pcolt94; 04-15-2018 at 10:07 PM.
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Old 04-15-2018, 10:18 PM   #5  
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I thought the block sensor fed the computer and the water pump sensor fed the dash?
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Old 04-15-2018, 11:06 PM   #6  
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Quote:
Originally Posted by johnanthonyhome View Post
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 ?).
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Old 04-16-2018, 08:06 AM   #7  
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If you get stuck working on this let me know. I have a GM Tech 2 clone and Snap-On MT 2500.
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Old 04-16-2018, 12:41 PM   #8  
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This is really good stuff, Paul. Thank you. I'll report back later tonight or tomorrow.
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Old 04-19-2018, 09:24 PM   #9  
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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.
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