High RPM is worse, HELP!

Steve491 · Nov 9, 2010

I've tried everything to get this car to run right and still a huge lack of high RPM and a severe miss over 4500.
New fuel pump
new filter
fuel pressure is at factory specs (40 psi)
new plug wires
new plugs
MAF cleaned with the proper cleaner
no vacuum leaks can be detected
no CEL
IMRCs were cleaned and seem to work fine
Intake is sealed up with new Ford gaskets
I have an OBDII interface for my laptop that reads most sensors and the short term fuel trim is going very high when it starts running like crap. When I push on it the reading is +35% and it lays down and misses bad. Im I correct in assuming thet means the computer wants to add fuel? I think it's trying to add, add, add.!
Some of you Mustang gurus please chime in here because I'm at the end of my rope. It's a good solid car that just needs something small fixed to make it a great running car again.
On another note, I don't know if the ECM has been flashed by a previous owner. How can I tell? If it has, can it be flashed back to factory?
Thanks in advance for any input, Steve

wmburns · Nov 10, 2010

Have you checked the fuel pressure when the problem occurs?

For the most part STFT changes too fast to be useful. What is the LTFT doing? And yes the positive STFT means that the PCM is attempting to add fuel.

Your MY Cobra uses a two speed fuel pump. What if there is a problem with the fuel pump relay that switches from low speed to high speed? Say for example, the Fuel pump is staying in the low speed mode. That would explain how it would run great at low RPM's but fall flat once the capacity of the low speed fuel pump was exhausted.

Steve491 · Nov 11, 2010

Thanks for your response. From other posts that you've responded to It sounds like you know what you're talking about.
I wasn't aware the car had a 2 speed fuel pump. I'm only able to check the fuel pressure in the driveway @ the rail. At idle it's about 28 and instantly goes up to 40 when it's revvved. Where is this relay? What triggers the second stage?
Thanks!

Steve491 · Nov 11, 2010

I looked on Rockauto.com and they listed 2 relays. One was a normal looking relay that would be in the fuse box and the other looked like a solid state component with a huge connector. Wonder which one applies to a 97 Mustang...

wmburns · Nov 12, 2010

The Fuel pump resister is listed as "RH front back part of fender". The wire colors are LG/V.

The fuel pump relay is listed as "By powertrain control module right hand side kick panel".

The PCM controls when the fuel pump relay changes. I suspect it is RPM based (3000+) and works similar to the IMRC.

Suggest installing a volt meter on the IFS switch in the trunk. Get a long set of leads so that the voltage can be monitored while driving. Use a known good ground. Make note of the voltage on the fuel pump. It should be obvious when the relay changes.

Note, this could also be a weak electrical connection not passing enough current/voltage to the fuel pump. Testing the voltage will let you know either way.

Steve491 · Nov 14, 2010

Installed a new fuel pump relay today and it clicks on around 3250 when the IMRCs are opening but the problem persists. I haven't checked the voltage in the trunk yet. The LTFT seems to be up to par, nothing out of the ordinary when I push on it.
Because it's so easy I decided to check the ignition system. Pulled the plugs and they all seem to look different. A couple are black around the insulators and a couple are gray. The plug wires are only a year old, factory Morotcraft, and one actually broke when removing it. I used dielectric grease around the boots on both ends and it still broke. Plugs were Autolite Platinum. I thought the coils were original but I found a sticker on the side "made in China". Go figure. Maybe I'm barking up the wrong tree??
It has to be something simple because the car doesn't smoke or tick, nor does it look to have been taken apart to any degree other than the normal maintenance over the years.
AAAhhhhh!!!!

wmburns · Nov 14, 2010

Try this old racer's trick. Run the car in a totally dark location. Look for the blue hue of escaping spark. Inspect the spark plugs wires for white spots which is evidence of arcing.

Steve491 · Nov 16, 2010

Thanks wmburns, My new ignition parts will be here on Thursday. Sometime I overlook the obvious, but the almost new wires were installed less than a year ago. Just tired of chasing what I keep thinking is a stupid problem.
I put the new Motorcraft wires on because the previous owner had some "hot rod" wires on it that were laying on the exhaust and all messy.
I'll figure it out eventually thanks to your help!
Kinda like a boat... The best 2 days are the day you buy it and the day you sell it.
In the mean time I'll keep on driving my trusty '95 F-150 with the original five-O.
-Steve-

Steve491 · Nov 18, 2010

Got my new coils. They are made in America, not OEM, but it's worth a shot. I've eliminated everything in the fuel system except the injectors themselves. Ignition is the focus right now because it's easy to do.
New plugs going in are trusty ole Autolite 764 copper. I'm not looking for duration, I'm looking for performance.
Charlotte Checkers hockey on Friday, Cobra and raking leaves this weekend.
-Steve-

wmburns · Nov 18, 2010

Think about using an injector cleaning and flow testing service such as injectorrx.com. I have had good luck with their service. Once cleaned, they are as good as new and much cheaper.

I had the injectors cleaned when I re-did the motor on my 2000 GT. Very interesting report. Showed exactly how much lower than spec the flow was. Also a large imbalance between the injectors. Once cleaned, the flow was up to specs and within a percent of each other.

Still think you should TEST the voltage at the fuel pump. With the age of your car, the possibility of a bad ground or other weak electrical connecting is very real. It would not take much resistance to make a big change in the available voltage at the fuel pump.

Have you done a compression test?

If still not able to find anything, consider a KB BAP.

Steve491 · Nov 21, 2010

Problem Fixed.!!

Well, it seems the Chinese coils were my problem. The only things I replaced yesterday were the coils and the plugs. Since I've tried a couple different sets of plugs before, it had to be the coils. I was chasing a fuel problem all along and it turned out to be an ignition problem, go figure. They seemed to have deteriorated gradually over the last year or so. Since most everything on the engine was stock I assumed the coils were stock and good. WRONG.! The car now runs up through the RPM range perfect including the factory hit @ 3250 when the IMRCs open and the fuel pump hits high gear.
Thanks to all, especially wmburns, for your input. Hopefully this thread will help some other frustrated Cobra owner chase down their problem.
Happy in Carolina, Steve

shoobs99svt · Nov 23, 2010

Glad to hear youre back on the road Steve... Ill continue to watch for a black "flash" to pass me on I485 lol

Cheers

Steve491 · Nov 23, 2010

Hey Shoob, Haven't heard anything from you since you ran your car down the Zmax dragstrip here in Charlotte. You won't see me flyin by until I get some new tires. Now that it runs good again I realize my tires are crap.

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