LT 4 idle and misfire.
04-07-2018, 08:13 PM
#1
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LT 4 idle and misfire.
Engine runs fine when cool. As it warms up idle surges from 800 to around 1500 back and forth. Occasionally a misfire in second gear but in 6th gear...pretty much a misfire when accelerating. Car has had new..Head gaskets, plugs, wires, water pump, clutch, pressure plate, rear main seal, fuel pump, distributor cap within the last year. Searched and read everything on here about this problem. I was going to start with idle air flow valve and work through throttle position sensors and EGR. But....all YouTubes show the “P” vin motor not the “5”. I don’t see the AIC on the throttle body of this engine. Did take it to Tire Kingdom today . Manager has always been helpful in my regular vehicles. They hooked it up to computer and said that no codes but it didn’t respond when they tried to open EGR and adjust idle...?? Is there a service manual that covers the LT4 and any suggestions. My first thought was vacuum leak but can’t here one. Will buy some carb cleaner tomorrow to spray around hoses to see...
04-07-2018, 09:53 PM
#2
Le Mans Master
LT4 does not have egr. If it's misfiring PCM should pick it up. If scanner can read misfire counter it can point you to which cylinder. Opti cap and rotor is a possibility.
04-07-2018, 11:05 PM
#4
Pro
Because the LT4 is a single year engine in the Corvette, the best reference manual is the 1996 GM Factory Corvette Service Manual and these can usually be picked up on this Forum For Sale or on Ebay. Get both volumes, its a two book set, red cover. I wonder if the surging is happening as it transitions from open loop to closed loop?
04-08-2018, 07:54 AM
#5
I had a misfire that would show up on my scanner but not throw any codes.
It would misfire the most in 6th gear under load. It turned out to be the rotor retaining scews being loose (one dropped out completely and was laying in the cap) in the optispark. I cleaned everything and installed a new GM cap, seal, rotor kit. That took care of the misfire. If you install a new rotor make sure you use some blue loctite on those retaining screws.
It would misfire the most in 6th gear under load. It turned out to be the rotor retaining scews being loose (one dropped out completely and was laying in the cap) in the optispark. I cleaned everything and installed a new GM cap, seal, rotor kit. That took care of the misfire. If you install a new rotor make sure you use some blue loctite on those retaining screws.
Last edited by grandspt; 04-08-2018 at 07:55 AM.
04-08-2018, 12:35 PM
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OptiSpark
I had a misfire that would show up on my scanner but not throw any codes.
It would misfire the most in 6th gear under load. It turned out to be the rotor retaining scews being loose (one dropped out completely and was laying in the cap) in the optispark. I cleaned everything and installed a new GM cap, seal, rotor kit. That took care of the misfire. If you install a new rotor make sure you use some blue loctite on those retaining screws.
It would misfire the most in 6th gear under load. It turned out to be the rotor retaining scews being loose (one dropped out completely and was laying in the cap) in the optispark. I cleaned everything and installed a new GM cap, seal, rotor kit. That took care of the misfire. If you install a new rotor make sure you use some blue loctite on those retaining screws.
04-08-2018, 12:49 PM
#8
The balancer removal was the toughest part of the job, it seems to rust up and doesn't want to be separated. If you do have to remove the balancer make sure you spread some antisieze compound on the hub and balancer so it will come off easier next time around.
04-09-2018, 06:34 PM
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Cleaned IAC valve
Took little aggravating bugger out and cleaned. Used the recommended GM cleaner. Lubed O Ring. Ohmed 1to 2 pins 0.43 resistance same on 3-4 pins. Reinstalled and test drive shows no change..?? I’m not discounting the Optispark but that’s a bigger deal and not sure...Anyway I’m trying...
04-09-2018, 11:40 PM
#10
Melting Slicks
Took little aggravating bugger out and cleaned. Used the recommended GM cleaner. Lubed O Ring. Ohmed 1to 2 pins 0.43 resistance same on 3-4 pins. Reinstalled and test drive shows no change..?? I’m not discounting the Optispark but that’s a bigger deal and not sure...Anyway I’m trying...
04-10-2018, 09:12 AM
#11
Le Mans Master
Wait, before we blame the opti for yet another problem it probably isn't causing, let's go back to the original post. Ron, you said your car runs okay at cold startup, but begins running badly after it warms up, right? So what does that tell us? Answer: it runs fine in open-loop PCM mode, but starts running badly when it enters closed-loop operation. That tells me it's more likely a sensor either going bad or actually sensing something bad. I'd want to data-log the air fuel and spark to see if I could identify something changing or flatlining after warm-up. I strongly, strongly doubt this has anything to do with the opti.
Last edited by MatthewMiller; 04-11-2018 at 11:32 PM.
04-10-2018, 12:05 PM
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04-11-2018, 10:00 PM
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Lt4
Truly hope no opti issues. Cleaned the Mass Air Flow Sensor today. Looked dirty as did IAC. Throttle body looked a bit dirty. Car says to have a KN air filter...I’ll check it out...After cleaning MAFS got code P1054. Car seemed better but just a bit. Came home and cleared code and hasn’t shown back. Funny thing is if I see a caution lite coming and clutch...idle is fine until car is at stand still...??
04-11-2018, 10:07 PM
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Thanks
Thanks for all help...I’m 64 and eyesite is not great...Just Trying...Takes me three times to do these things now...But LT 4 is torqued...!!
04-12-2018, 02:05 AM
#17
Tech Contributor
Check the impedance on the injectors. They should be around 12 ohms. Injectors going bad drive the PCM crazy (via the o2 sensors) once it enters closed loop. Have seen this on several C4s and now that the LT4 is 22 years old, it’s bound to happen on some of them.
04-12-2018, 05:41 AM
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Engine runs fine when cool. As it warms up idle surges from 800 to around 1500 back and forth. Occasionally a misfire in second gear but in 6th gear...pretty much a misfire when accelerating. Car has had new..Head gaskets, plugs, wires, water pump, clutch, pressure plate, rear main seal, fuel pump, distributor cap within the last year. Searched and read everything on here about this problem. I was going to start with idle air flow valve and work through throttle position sensors and EGR. But....all YouTubes show the “P” vin motor not the “5”. I don’t see the AIC on the throttle body of this engine. Did take it to Tire Kingdom today . Manager has always been helpful in my regular vehicles. They hooked it up to computer and said that no codes but it didn’t respond when they tried to open EGR and adjust idle...?? Is there a service manual that covers the LT4 and any suggestions. My first thought was vacuum leak but can’t here one. Will buy some carb cleaner tomorrow to spray around hoses to see...
04-13-2018, 03:47 PM
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Doesn’t start rough idle till 0 mph shows.
I tried this several times today. Approaching a stop light I shifted into neutral and coasted to stop. Idle is fine at 800 all the way down to 0 mph then starts bouncing between 800 and 1000. Better than before but still not right. Idles fine at 2 mph as long as car is rolling. Does something change in ihe idle circuit when speedometer hits 0..?? (Code I had was P0154, mistyped it but it’s cleared).
04-13-2018, 04:00 PM
#20
Le Mans Master
there is no "idle circuit" a vehicle speed reading above 0 will effect pcm's command of IAC. TPS at idle? Min Air Rate - is throttle body clean? Vacuum leak? the T on right side of intake and vacuum hose near battery are common failure areas.
Last edited by Kevova; 04-13-2018 at 04:00 PM.