Random Misfire Diagnosis

[josiah01]

I have read a LOT of info on this topic. Random misfire seems to kind of be a "car-specific" deal, and I'm having trouble diagnosing it for certain. Thank you in advance for your help. And now for the car:

97 GSX 5spd, lightened flywheel
1G block, 2G head, unsure which CAS, guessing its a 1G (bought the car already built)
T-28 turbo, custom FMIC, all hard piping, no obvious boost leaking
Stock fuel pump, injectors ( i don't boost over 14lbs set with MBC)
No tuning/logging software (yet)
Brand new plugs (didn't solve the problem)
8.5mm wires

Symptoms:
CEL turns on randomly, can't force it, not always on, after i reset it it takes a "while" to come on again.
9 out of 10 times it comes on in 5th gear around 60mph, when gently accelerating
Engine tone changes when CEL throws, when I accelerate the engine begins "thumping" similiar to the "machine gun" noise other users have described. Once the CEL goes, the car becomes unbearably slow, accelerates very slowly, still getting boost on the gauge but not the typical response (ECU retarding timing or something?) I don't know which code it throws (need to find somebody with a code reader) The car has done this periodically since I bought it, so I know it isn't a mod I've recently done (the FMIC and sparks are new).

Any thoughts are appreciated. I will provide any more detail that I can. Hopefully I described the symptoms appropriately. Thanks!

 

[Old Mitsu Tech]

Therein lies the problem. Give us the code and the description of the code you are receiving. Is there bogging or missing associated with this? Give us some more specific symptoms. Good Luck

 

[BISHILVR]

Sounds like you are dropping a cylinder to me. Maybe you have bad plug wires??? Autozone scans for FREE (hint, hint).

 

[Northbendtech]

this is a common problem with the 6 bolts swaps. install a 10k potentiometer inline with the barometric pressure sensor wire feeling into the ecu plug. (orange wire I believe) turn it half way. if your problem doesn't go away turn it a little more. basically the ecu doesn't like the signal from the 1g CAS and it causes a missfire code. by adding resistance to the Barometric circut you put it out of range and the engine will not check for missfires, thus solving your problem.

 

[xcz504d]

I just recently had a similar problem, I couldn't figure out what it was for the life of me... My CEL kept coming on after about 30 miles, and I would get random misfires (usally under boost over 10 PSI)... I thought maybe my exhaust valves were gummed up, and I went through a WORLD of troubleshooting... but I failed to check the little things first...

My number 2 spark plug wasnt gapped properly... Changed the plugs, and presto! Oh... the CEL came on after 30 miles because of my O2 sensor... Good luck with your problem.. Check the little things first...

 

[Highwayrun]

You have the exact same problem as me! I'm not sure how to fix it either, but when it happens, if you push the clutch in and let the RPMs drop to idle, the problem will go away. The CEL will clear itself later, but at least you'll keep that big rig from rear-ending you.

(I think my problem may be in the #4 plug wire, since they look like the original wires. )

 

[xcz504d]

A little maintenance will go a long way Another neglected maintenance story for you guys... CHECK YOUR TRANSFER CASE LUBRICANT!!! They leak!!! I didn't read about that until it was a hunk of grinded metal!!! It sucked trying to loosen rusty exhaust bolts to get the transfer case off....

Learn from my mistakes...

Ohh... one more to learn from... I had a mazda mx-6 once... fast little bugger, easily would pull mid 14's all day long and it was naturally aspirated! I went on road trip after road trip... all over... a good 8000 miles in a month or so... I was about 5 miles from home, and I was at a stop light... I noticed my temp gauge creeping up... I turned my music down to hear a ticking sound... I immediatly recognized the ticking sound... pulled over... that was the last time that engine turned over....

CHECK YOUR OIL!!!

My neglected maintenance cost me $3000 on the mx-6 and so far $120 on the talon (and alot of my precious time troubleshooting!) But my transfer case should be here in a week or so Finish up on my interior (its getting completely re-done, headliner, carpet, upholstry, everything is scattered around the house and the wifes not happy )

In fact... I would venture to say most people have problems on this site due to lack of general maintenance...

 

[96whgsx]

I have read a LOT of info on this topic. Random misfire seems to kind of be a "car-specific" deal, and I'm having trouble diagnosing it for certain. Thank you in advance for your help. And now for the car:

97 GSX 5spd, lightened flywheel
1G block, 2G head, unsure which CAS, guessing its a 1G (bought the car already built)
T-28 turbo, custom FMIC, all hard piping, no obvious boost leaking
Stock fuel pump, injectors ( i don't boost over 14lbs set with MBC)
No tuning/logging software (yet)
Brand new plugs (didn't solve the problem)
8.5mm wires

Symptoms:
CEL turns on randomly, can't force it, not always on, after i reset it it takes a "while" to come on again.
9 out of 10 times it comes on in 5th gear around 60mph, when gently accelerating
Engine tone changes when CEL throws, when I accelerate the engine begins "thumping" similiar to the "machine gun" noise other users have described. Once the CEL goes, the car becomes unbearably slow, accelerates very slowly, still getting boost on the gauge but not the typical response (ECU retarding timing or something?) I don't know which code it throws (need to find somebody with a code reader) The car has done this periodically since I bought it, so I know it isn't a mod I've recently done (the FMIC and sparks are new).

Any thoughts are appreciated. I will provide any more detail that I can. Hopefully I described the symptoms appropriately. Thanks!

Exact thing happened/happening to me. I recently (within 150 mi.) got a 6 bolt swap done and my car feels really sluggish and rumbly on acceleration. This just happened like 2 days ago. BTW, the CEL code I got was P0300, random misfire code.

I have a question about using that 10k potentiometer. Would this mod eliminate both the CEL light and the sluggish behavior of the car?

 

[BISHILVR]

It will stop the ecu from checking certain things that cause the cel to set. The problem is that it's a "bandaid" fix, since your are skewing the sensor inputs to the ecu which is not always a win, win.

 

[Empyrials]

now granted i've not done the 6 bolt conv yet, however i've had the same prob and it SUCKS.

I had to replace both coils to get it fixed. I replaced one of them and it changed the speed in which it detected the Random Missfire, so replaced the other one and It was gone!!! just another item to look into i guess

 

[ddavisaf]

DSM Link takes this problem away quick

 

[Numba1cuban]

Ok i got the same problem and i see that everyone says that dsm link takes the problem away. Does this mean that my car will stop misfiring or that its going to misfire but i wont get a CEL? Because if it is just to get rid of the CEL you might as well go in back of the dash and take of the bulb.

 

[jasteel13]

this is a common problem with the 6 bolts swaps. install a 10k potentiometer inline with the barometric pressure sensor wire feeling into the ecu plug. (orange wire I believe) turn it half way. if your problem doesn't go away turn it a little more. basically the ecu doesn't like the signal from the 1g CAS and it causes a missfire code. by adding resistance to the Barometric circut you put it out of range and the engine will not check for missfires, thus solving your problem.

thats right its the 2g head on the 1g block... does it almost everytime...

i know their was a tech write up on this some where if you have it could you please post it up