LT1 Missing/Surge
Thread Starter
7th Gear
Joined: Apr 2021
Posts: 7
Likes: 1
From: Bethlehem Ga
LT1 Missing/Surge
Hi everyone, I'm new here and to the LT1 world. I purchased a 1993 C4 with a blown head gasket. It had a bad miss that I figured was due to the head gasket being bad and fowling a plug. After getting everything back together I still have a miss, worse when warmed up than cold. The idle is fine one day and then a bit higher another. I have checked the temps on the exhaust and found the left side is higher than the right. After about 5-8 mile cooling temp hits 175-180 when the miss gets worse, like 1 cylinder is out. After another 5 miles or so temp is around 224-227. ECM will give me Code 12 to confirm its working, but no other codes come up and CEL has not come on at all. Long story shorter I have done the following:
Machined and rebuilt the heads, new seals, new gaskets etc. All put back together to GM specs
New MAP, IAC, TPS, Cooling Sensor, Opti-Spark, Plugs, Wires, Coil, Bosch Injectors, O2's, EGR, Vacuum Lines, Brake Booster, Fuel Filter
I have read several posts about similar issues but nothing that matches what is happening here.
If anyone has any ideas on what I can look at next it would be greatly appreciated being I'm at a loss with what the problem could be. Thanks
Machined and rebuilt the heads, new seals, new gaskets etc. All put back together to GM specs
New MAP, IAC, TPS, Cooling Sensor, Opti-Spark, Plugs, Wires, Coil, Bosch Injectors, O2's, EGR, Vacuum Lines, Brake Booster, Fuel Filter
I have read several posts about similar issues but nothing that matches what is happening here.
If anyone has any ideas on what I can look at next it would be greatly appreciated being I'm at a loss with what the problem could be. Thanks
Drifting
Joined: Apr 2020
Posts: 1,925
Likes: 610
From: Orange, CA
Where'd you get the replacement optispark from?
Did you replace the ICM? And if so, when you replaced it, did you make sure to use new thermal paste on it?
Just throwing the usual suspects out there.
Did you replace the ICM? And if so, when you replaced it, did you make sure to use new thermal paste on it?
Just throwing the usual suspects out there.
Thread Starter
7th Gear
Joined: Apr 2021
Posts: 7
Likes: 1
From: Bethlehem Ga
I purchase the opti spark from Jegs
Part Number: 555-40010
I have not replaced the ICM due to the comments about it failing and the engine shutting down or not restarting. I have not had that problem.
Drifting
Joined: Apr 2020
Posts: 1,925
Likes: 610
From: Orange, CA
I'm sorry to say, but it's entirely possible that the opti you received is no good. There are only really three trusted sources for optisparks (optisparksolutions on eBay, Petris Enterprises, and Sac City Corvettes), and getting one from anywhere else is a toss-up.
That said, since it's a '93, it's also possible that the ECM is bad. Unfortunately I personally am not familiar with the tools to work with a '93 (since I do not own one), so I cannot help you directly in diagnosing your issue. Hopefully someone with more experience can jump in. Do you know any other history of this car? Is the miss it has now exactly the same as before you threw parts at it, or has the miss changed since?
That said, since it's a '93, it's also possible that the ECM is bad. Unfortunately I personally am not familiar with the tools to work with a '93 (since I do not own one), so I cannot help you directly in diagnosing your issue. Hopefully someone with more experience can jump in. Do you know any other history of this car? Is the miss it has now exactly the same as before you threw parts at it, or has the miss changed since?
Le Mans Master
Joined: Aug 2015
Posts: 6,090
Likes: 1,972
From: St. Charles MO
There are two coolant temp sensors: one in the passenger head and one in the water pump. Which one did you replace? If you just did the one in the head, that's not the one that matters. The ECM only reads the one in the water pump.
Thread Starter
7th Gear
Joined: Apr 2021
Posts: 7
Likes: 1
From: Bethlehem Ga
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Thread Starter
7th Gear
Joined: Apr 2021
Posts: 7
Likes: 1
From: Bethlehem Ga
5-2-21 Update. Took some time yesterday trying to track down the problem. So far I tested all injectors (That are new). Check for signal to all with a signal tester. Put a timing light on all plug wire to check for consistent signal. All seemed good however when I unplugged #3 and #7 injector I had no change in the engine. So I will now check the plugs that are new NGK Platinum to see if I somehow have 2 bad plugs. It has been suggested that I use E3 plugs, if anyone has any other ideas as to what plug the LT1 likes or dis-likes I'm all ears.. Have a blessed Sunday y'all..
Last edited by Hopz24; May 2, 2021 at 09:56 AM.
Drifting
Joined: Apr 2020
Posts: 1,925
Likes: 610
From: Orange, CA
5-2-21 Update. Took some time yesterday trying to track down the problem. So far I tested all injectors (That are new). Check for signal to all with a signal tester. Put a timing light on all plug wire to check for consistent signal. All seemed good however when I unplugged #3 and #7 injector I had no change in the engine. So I will now check the plugs that are new NGK Platinum to see if I somehow have 2 bad plugs. It has been suggested that I use E3 plugs, if anyone has any other ideas as to what plug the LT1 likes or dis-likes I'm all ears.. Have a blessed Sunday y'all..
Thread Starter
7th Gear
Joined: Apr 2021
Posts: 7
Likes: 1
From: Bethlehem Ga
6/21/21 Update: I have tested the injectors, fuel psi is 50-52 I can smell rich exhaust from the cylinders not firing. I replaced the NGK plugs with E3 plugs. When I removed all the NGK plugs #3 and #7 looked new still. Tested compression on #3 and #7, 150 psi. I ran the car for a few minutes and still had the same issue with 3/7, plus looked clean still when I pulled them. I put a HEI spark tested on the plug wire and I get spark. I was totally thinking the new Opti Spark was junk but now I'm even more puzzled as to what the real problem could be. Any ideas would be helpful. Thanks fellow C4'ers
Pro
Joined: Dec 2017
Posts: 534
Likes: 162
From: Omaha, NE
Missing
I had a 92 automatic Vert that had a miss and a rich smelling exhaust. I have a scanner and didn’t see any problems and I didn’t have any codes. I changed out my plugs, wires and my opti, with no change. So I unplugged each injector and noted any change in the engine idle. Turned out that I had 2 bad injectors,#2 and #8. The shop that I ended up taking it to tried moving both injectors to the other side of the engine to verify that they were bad. I had the original injectors in the car and they found 2 new ones ( not refurbished) and it fixed my issues. Kevin
Thread Starter
7th Gear
Joined: Apr 2021
Posts: 7
Likes: 1
From: Bethlehem Ga
We found it finally!! The owner that I purchased it from had #3 and #7 wires switched on the optispark. Took some thinking outside the box to find it and small hands to switch it back right.. Thanks y'all for all the ideas..