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1994 Cobra With Serious RPM Issues.

2K views 21 replies 7 participants last post by  Gkperformance 
#1 ·
Hello all! I've come to the forums to get advice from you guys on an RPM issue. I've got a 94 Cobra with just a CAI on it. Pretty much factory stock. It started having a loss of RPM and then it would gain it right back causing the car to buck back and forth. Over the months, it would occur once every two weeks or so and then it progressively got worse. I started replacing parts through the ignition system and every component I replaced was corroded or just worn out. I swapped the fuel filter since, at the time, it only pulled the trick under a light load and higher than 2500 RPM didn't have loss but it did nothing. Since then, I have replaced the cap and button, the ignition coil, the plugs and wires and finally the engine control module when the car would just constantly back fire and die. Also, the more the engine heated up, the worse the RPM loss would be. I'm at the end of my knowledge and googling skills so any advice would be appreciated!:thumb: I'll add a video of the tach after I install the new ECU today. Thanks for the time!

---------- Post added at 06:52 PM ---------- Previous post was at 06:38 PM ----------

Here's the link to the tach video. Thanks!



 
#2 ·
Hello all! I've come to the forums to get advice from you guys on an RPM issue. I've got a 94 Cobra with just a CAI on it. Pretty much factory stock. It started having a loss of RPM and then it would gain it right back causing the car to buck back and forth. Over the months, it would occur once every two weeks or so and then it progressively got worse. I started replacing parts through the ignition system and every component I replaced was corroded or just worn out. I swapped the fuel filter since, at the time, it only pulled the trick under a light load and higher than 2500 RPM didn't have loss but it did nothing. Since then, I have replaced the cap and button, the ignition coil, the plugs and wires and finally the engine control module when the car would just constantly back fire and die. Also, the more the engine heated up, the worse the RPM loss would be. I'm at the end of my knowledge and googling skills so any advice would be appreciated!:thumb: I'll add a video of the tach after I install the new ECU today. Thanks for the time!

---------- Post added at 06:52 PM ---------- Previous post was at 06:38 PM ----------

Here's the link to the tach video. Thanks!



1994 Mustang Cobra RPM Issue - YouTube
fuel pump itself? fuel injectors? clogged exhaust? I've heard even the tranny can cause issues like this.
 
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#7 ·
Doublecheck the plugs, check for any vacuum leaks, check for any codes, and just for the hell of it run a compression test. Check the grounds. May be none of the above, but it never hurts to check. Fill the tank lately? Maybe its bad gas. The backfiring is usually timing related. Check that as well.
 
#8 ·
Thanks for pointing me in some directions guys! I'll have to get to work trying to run this stuff down. And it may be worth noting that the driver side exhaust is running around 400 degrees whereas the passenger side is at 250 degrees. It seems a bit odd to me to have a 150 degree difference. It slipped my mind until I was sitting here after work and making this reply!
 
#9 ·
My last Mustang had a similar symptom occur related to exhaust temp,but it showed more than 150° difference.It ran rough too,but it was a lot more noticeable than what your car is doing.I was told it was a partially clogged converter,but I done a compression test and found out one of cylinders was at 95psi while the others were at 160psi.Long story short,one of my valve spring keepers had popped loose from the valve stem and the valve wasn't seating completely.I'm not sure how the engine made it from Panama City,Fl to Tn but it did.So a compression test mentioned by fb93 is a good idea.Getting too much fuel dumped on one bank of the engine can cause the temp difference. If fuel isn't getting burned sufficiently on the suspect bank due to excess fuel,weak ignition component, etc the raw fuel will heat the converter up.If a injector was leaking or o2 sensor is reading lean causing the ecm to richen fuel on that bank in order to correct the lean or false lean condition, that would also heat the converter up.If you run the car for 15-20 minutes,shut it down,wait 10-15 minutes then pull the spark plugs,a wet spark plug would indicate a leaking injector.A leaking injector normally causes a hard/no start condition after the engine has been driven,shut down and then a restart attempted,but it could also start fine too depending on how bad the injector is leaking.
 
#10 ·
I got home from work today and there was a massive storm blowing up so I quickly did the screwdriver to ear test on the injectors I could reach with it and I could hear the steady clicking. But I did decide to stick my hand to the exhaust on both tips and I could feel small puffs of cold air. From my searching, that's a symptom of a clogged exhaust like Cobra_Commander said but my setup is factory headers, x pipe, Flowmaster 40's and then tips so no cats to clog up.
 
#11 ·


perhaps try this? the mufflers can still get clogged, though its less likely since usually they get clogged by the cat coughing up garbage.

curious to see what an OBD-II tuner or one of those dongles says if you can do some data recording.

also maybe do this for good measure

 
#14 ·
All kitties around my house have been accounted for so there's no cat clog lol. I took the MAF off today and cleaned the element and put it back together and took it down the road and it idled better but the issue came back. I unscrewed the component behind the MAF (air temp sensor I think. It's neon orange.) and it was pretty dirty and the connectors had some corrosion so I cleaned it as well and put it back together and crunk it up and the CEL flashed on and I thought "A-HA! Now the computer can give me a code." But I looked and saw I forgot to plug in the MAF! It ran just the same as when the MAF was plugged in but no CEL when the MAF is plugged up. Only difference I could tell was a near stall when I first cranked the engine but that's it. Thank you guys for sticking with this thread and giving me some tips! Anything past standard oil change, spark plugs and lug nuts is past me without having to watch a YouTube video and I can't find a "how to" video for this problem lol.
 
#16 ·
The wires were good justAbullitt. I had another guy hook up a new Snapon computer to it and it said the MAF was out of parameters so I swapped it out after the cleaning didn't work. It didn't help but the computer says the MAF is now in the tolerances lol. Now it just hovers around 200 rpm and then dies. I rechecked the plugs and wires and made sure the firing order didn't magically get crossed up somewhere and when I pulled the plugs, they had lots of gas on them and the exhaust smells of gas too. For some reason its dumping more fuel than needed and the fuel pump is spot on and the computer told us the injectors were operating like new. This is a real humdinger to me...
 
#17 ·
probably isn't it but maybe.. is the throttle body clean? confirmed to be working? not stuck closed. if its clean push on the pedal with the car off and see if it moves, I believe its a mechanical TB right? With a idle valve (check its clean).
 
#19 ·
Have you pulled the intake tube off and looked at it? Crud could be all up behind it too. What I am thinking is, maybe the air coming in is restricted, so when you push on the throttle, even more gasoline is injected but there is no air coming in. It could be at the throttle body or at the intake manifold. Either physical crud, or the throttle body not moving right.

Wont hurt to look if you haven't. Taking off the intake manifold will take more time, but checking the throttle body is fast.
 
#20 ·
Sounds like a bad O2 sensor..
Also, did you disconnect the battery after replacing the MAF Sensor?
The PCM will keep data of the fuel map that was running when the bad MAF sensor was in the car, to protect itself in case of a failed sensor.. If you disconnect the batter, and leave it disconnected for 5-10min it should clear out the fuel maps.. which may also help your issue.. But the Flooding, sounds like a bad O2 sensor..

Bad O2 sensor would also explain the exhaust heat differences, the flooding, and the flooding would explain the hesitation..
 
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