Misfire at idle and under load....no codes

[Mentos]

Another misfire thread! I've searched and found some helpful info, but I wanted to throw a question out there.

My daily driven 140K mile '97 Cobra (stock minus an O/R x-pipe and mufflers), just started with a misfire today. I drove it all day yesterday without issue. I started it this morning, and moved it into the garage which took all of 30 seconds and ran fine.........then I come back out an hour later and it sounds like a cylinder is misfiring. All of a sudden sort of thing. Never had an issue till today.

Tune up was about 4 months ago.
New fuel filter, air filter, plugs (Autolite coppers properly gapped), and some parts store brand wires. I pulled the plugs today, all look great.

Odd thing is I'm getting NO codes at all (wish it would throw something so I could check). I drove it around the block and it misses under load. Idles a little lower than normal as well....almost to the point where it might die.

Suggestions appreciated. I'm a pushrod guy, not as knowledgeable with the mod motors. ...but I'm learning!

 

[ademily87]

I'd start with a compression test

 

[Mentos]

I'd start with a compression test

Thanks. Will do, gonna see what tools the parts store will loan me today.

 

[Mentos]

A friend of mine mentioned a possible issue with the coil packs. I assume those shoud be tested too. Not sure of the 'life span' of those.

 

[ademily87]

A friend of mine mentioned a possible issue with the coil packs. I assume those shoud be tested too. Not sure of the 'life span' of those.

That too. Could try letting the car idle and unplugging one coil pack at a time and listen for a change in the idle.

 

[ZeroDCX]

That too. Could try letting the car idle and unplugging one coil pack at a time and listen for a change in the idle.

The OP's car is a 97 that uses coil packs (two coil packs with four posts that fire two cylinders at the same time), not COP. Unplugging one coil pack at a time will prevent four cylinders from firing. This would be pointless as, there would definitely be a change in idle while only running on four cylinders.

 

[ademily87]

The OP's car is a 97 that uses coil packs (two coil packs with four posts that fire two cylinders at the same time), not COP. Unplugging one coil pack at a time will prevent four cylinders from firing. This would be pointless as, there would definitely be a change in idle while only running on four cylinders.

Oh yea jk lol haven't had my 98 cobra since 2009 and slipped my mind

 

[Mentos]

Thanks for the above input folks. I was researching how to test the coil packs today. I've come across some good info I believe. I'm going pull out the volt meter to give it a whirl. Might clean the MAF just to be on the safe side, although their are no codes.

Also gonna check all the hoses.....no apparent vacuum leaks, but you never know.

This morning I actually had to drive it back to my house since I was at my girlfriend's this weekend. It did 'okay'...just feels like its got a constant misfire in one cylinder.

I'll post up my results in a bit.

 

[Mentos]

Tested the resistance of the coil packs per some instructions I found online (along with the Haynes auto manual). Both test within spec.
Cleaned the MAF wires with the CRC MAF spray.
Checked all of the obvious hoses and sprayed from CRC spray around the intake and behind it (also around firewall). No obvious vacuum leaks. My dad has a vacuum gauge, may see if I can grab it this weekend.

I've got a friend with a set of new wires, gonna swap them this weekend to see what happens. ....if no improvement, then I'll try a compression test.

Also, while standing behind the car while it idles, its definitely an audible. Might 'attempt' to capture some video of the idle tomorrow after work. For now its quitting time. Thanks for the suggestions.

 

[Mentos]

Cold start... any thoughts?
[youtube_browser]hmiZ10eWFVw[/youtube_browser]

 

[joe111599]

Cold start... any thoughts?
[youtube_browser]hmiZ10eWFVw[/youtube_browser]

I had the same issue a couple months ago. Turned out to be a bad injector. When the car was cold it ran perfectly fine then when it warmed up to normal operating temp it started running like CRAP.

The way I found out it was a bad injector was I took a long screw driver and touched the end on each injector and pressed the end of the handle against my ear. You should hear a clicking sound(with car running)...actually pretty loud if the injector is working correctly. I checked all of mine and had one that was dead. No clicking. So I bought a new injector and threw it in. Problem solved!

Hope that helps. If it's not that then it could be a million other things. That's what pissed me off the most was I wasn't getting a check engine light either. :fm:

 

[Mentos]

I had the same issue a couple months ago. Turned out to be a bad injector. When the car was cold it ran perfectly fine then when it warmed up to normal operating temp it started running like CRAP.

The way I found out it was a bad injector was I took a long screw driver and touched the end on each injector and pressed the end of the handle against my ear. You should hear a clicking sound(with car running)...actually pretty loud if the injector is working correctly. I checked all of mine and had one that was dead. No clicking. So I bought a new injector and threw it in. Problem solved!

Hope that helps. If it's not that then it could be a million other things. That's what pissed me off the most was I wasn't getting a check engine light either. :fm:

Thank you for your response.....and its quite funny how right you are. Yesterday, I started it up, casually took it around the block for about 10 minutes. I got back home, still running weird, still no CEL. I decided to try the codes anyway, sure enough one was pending... P0302 ....cylinder 2 misfire. My friend with the new wires came over and I threw those on, just to see if the results change. Of course no change, but I took it around the block again......finally the CEL came on with the P0302. I immediately came back and pulled the #2 plug......looks good, no fuel odor or anything. I then did the screw driver test on the injector....which is in a 'barely reachable' spot on the #2 cylinder. I compared the 'clicking' sound to the others I could reach. #2 injector is definitely not as audible.

So...I'm going to check the resistance of all the injectors for comparison. I guess its time to pull the intake. Never pulled one of a 4.6 cobra. I assume I only need to pull the upper part of the intake. Difficult task?

 

[Mach828]

I'd check fuel pressure and fpdc to rule out the fuel pump first.

 

[Mentos]

I'd check fuel pressure and fpdc to rule out the fuel pump first.

Good call, maybe a parts store will have a gauge I can rent.

I don't suppose I could use any schrader valve gauge, could I?
Also, is there a 'backyard' mechanic way of test FPDC?

 

[ZeroDCX]

Thank you for your response.....and its quite funny how right you are. Yesterday, I started it up, casually took it around the block for about 10 minutes. I got back home, still running weird, still no CEL. I decided to try the codes anyway, sure enough one was pending... P0302 ....cylinder 2 misfire. My friend with the new wires came over and I threw those on, just to see if the results change. Of course no change, but I took it around the block again......finally the CEL came on with the P0302. I immediately came back and pulled the #2 plug......looks good, no fuel odor or anything. I then did the screw driver test on the injector....which is in a 'barely reachable' spot on the #2 cylinder. I compared the 'clicking' sound to the others I could reach. #2 injector is definitely not as audible.

So...I'm going to check the resistance of all the injectors for comparison. I guess its time to pull the intake. Never pulled one of a 4.6 cobra. I assume I only need to pull the upper part of the intake. Difficult task?

It's not difficult at all. Remove the STB, intake tubing, throttle cable bracket/throttle body, un-bolt the EGR tube, DPFE Sensor, IAC (Might as well clean it too), and other sensors. Be weary of the various gaskets, as you do not want to damage these, they are reusable (IAC, EGR, TB). Remove a couple vacuum lines that are attached to the intake lid. Then unbolt the lid and lift up. The upper to lower intake gasket is also reusable. Less than 10 minutes.

 

[Mentos]

ZeroDCX..........thank you for the input!

After looking it over earlier, it only seems to be a few bolts holding the upper lid on. I assume upper lid gasket is reusable as well. Will start pulling it apart tomorrow afternoon.

 

[ZeroDCX]

Yes it's re-usable.

If you're going to go through the trouble of taking off the upper lid. Then you should OHM out the injectors to check for big deviations between the set's resistance. If cylinder two's injector is greatly different in resistance versus the other seven then you've got a failing injector.

Also I would swap out cylinder two injector with another on the driver's side to see if the misfire code follows over afterwards. If so then you've isolated your problem. Assuming that the injectors passed the resistance test. Good luck.

 

[Mentos]

ZeroDCX, thankyou again for the advice. I'll report back with my findings.

 

[Mentos]

Pulled the upper lid, measured the resistance of the injectors, and number 2 is definitely dead. Got another ordered, hopefully I will have it in tomorrow.

 

[Mentos]

Little update, but nothing productive:

Ordered correct fuel injector from parts store yesterday, then go to pick it up this morning.....and its the wrong injector in the correct box.
Turns out its a GM injector for an S10. So I ordered another of the correct part, and will pick it up tomorrow morning. Probably won't get put in the car tomorrow though, since my day is booked solid.

I shopped around, and no one has the injector in stock, so I guess I'll wait another day.

The upside is now, I'm heading over to my cousin's place for a bit to cruise around in his beastly '71 Camaro autoX car. I never get tired of the LTs, X pipe, and dumped Borla XR1s on that thing.