View Full Version : Engine pulsing / hesitation

06-14-2003, 05:04 PM
My '97 Cobra has a stock engine. It has a pulsing or intermittent hesitation under full throttle above 5000 RPM. I notice it in second gear, but it may also happen in the higher gears, but I haven't had the opportunity yet.

It had 64,000 miles on the original Motorcraft spark plugs, which were noticeably worn. I changed to NGK TR55 plugs and the problem didn't go away. I've used injector cleaner in the tank, but also no improvement.

The engine runs smoothly and strongly otherwise.

The plug wires are original as are the injectors and ignition.

Anyone else experienced this?

06-15-2003, 10:13 AM
Same problem here just in 2-3000 rpm range. I put the car on the scanner and got a code message my #8 cyl was misfiring. Trying to trace problem as I type. Will let you know.

06-15-2003, 11:56 PM
I ran a compression test on all 8 cylinders when I changed the plugs. All were within 5psi of each other, and all of the original Motorcraft plugs looked identical, not too rich, not too lean...

I'm wondering whether I'm starving for air with the stock air intake. Any reports of the rubber intake tube between the MAF sensor and the throttle body constricting under hard acceleration? It is flattened to clear the shock tower brace. I wonder if it can collapse at that point. I don't currently have the opportunity to get the car to a dyno to test this theory. I was planning on fitting a cold air intake anyway, so maybe now's the time to make the purchase.

Any advice on a cold air induction system that mounts through the fender, uses the stock MAF sensor and connects to the stock throttle body? I've seen the MAC ones, but they look like they'd conduct more heat than the stock plastic/rubber. The prices seem to vary widely from vendor to vendor. I don't care about chrome dressups under the hood; I'd prefer to maximize performance.

06-16-2003, 12:18 AM
I don't believe it is a collasping intake tube...the engine doesn't suck that hard for air.

Have you changed your spark plug wires? Under a high-load (like in the upper rpms) if they are going bad you'll get the problem you are experiencing.....

06-16-2003, 12:22 AM
No I haven't, Tom, and they're still the originals with 64k on them. Should I replace them with Ford, or go aftermarket?

06-16-2003, 08:18 AM
That's up to you. I am currently using FMS 9mm wires and have used MSD 9mm wires as well. I've been happy with either.

With that many miles it's really time to change them anyways.....and hopefully it's what's causing your problems.

norcal cobra
06-17-2003, 01:06 PM
hey it could be your imrc motor the gears on the motor might be worn

06-17-2003, 02:53 PM
Well, until just now, I didn't know what an IMRC motor / plate was, nor that I had them. My car has 64k miles on it, the engine is completely stock and the engine looks to be in good condition. Should I expect the IMRC motor gears to be worn already?

I see that there are IMRC delete kits available. I also see that there was a change between 1997 & 1998. What are the pros / cons in deleting the IMRC?

Is there a way of lubricating the plates without pulling the intake manifold?

PS. I'll be installing new plug wires this weekend to see if the problem goes away.

06-19-2003, 12:21 AM
I had the same problem with hesitation/surging.
I used a Snap-on scanner and narrowed the problem down to fuel mixture adjustment switching on the left engine bank. Turned out to be my left upstream o2 sensor. Have you put your car on a scanner?

06-19-2003, 09:39 AM
Thanks for the reply.

No, I haven't had the car on a diag, but it's not a bad idea to do this occasionally, I imagine. Does it need to be an SVT dealer? We don't have one in my area, but I'll do a search for local shops that may have this capability.

Did you have to replace the O2 sensor? What were you charged for the scan?

06-25-2003, 11:21 PM
I changed the stock plug wires to FR 9mm & the problem went away!!

Thanks to all.

White Venom
06-26-2003, 12:38 AM
i have the same problem but not all the time i have noticed it in 3rd gear