High end hesitation

[toms97cobra]

I am new to the 4 valve motors and I am hoping you guys can teach me something here. I have a completely stock 1997 cobra 34000 mile, having an issue with the high end power. At WOT above 3000 rpm I can feel the car hesitate sporadically. I can feel the IMRC's open and it pulls but seems to go flat for a second, once or twice before 6500 rpm. From what I have read the IMRC's open at 3250ish. The defiantly open. Now also read its harder to tell in first and second, witch I find true feel it most in 3rd and 4th. Any ideas on how to go about this or should I not worry about it. I just put new plugs in it gapped at 54 they are the maximum motor sport OEM recommended replacement platinum plugs. I have read to reduce the gapping to about 45 on a stock motor. Is there a way to test the IMRC controller to see if it is holding the IMR's open, but from what I have read they either work or don't work. Any help would be cool .... I love the car and I am a little ocd on how it runs..... thanks guys

 

[GRUSE]

It sounds like the IMRC's are gunked up,even if they are opening 96 & 97's have the aluminum ones that get dirty a little quicker then the 98's.Plenty of threads on how to clean IMRC's .
If you can,return the platinum plugs and get some NGK TR55 or Autolite 764. Copper plugs>Platinum.
Does your car have MDS coils or wires?if yes ditch them for OEM coils.How old are the wires?Replace with FRPP 9mm when you change them.
Another thing could be old or clogged fuel filter.

 

[Cowtown]

If it's above the IMRC opening point I'd probably recheck your plug gap and of course the plugs. I also went on mine and replaced the original coils. My plugs were also gapped really badly, I don't know if whomever did them last just thought they went right in the way they were or read the completely wrong gap specs. That alone was the biggest hesitation fix on my side. They functioned but seemed to be a reasonable thing to look at swapping on a 18+ year old car. Mine revs nice and smooth up there now, aside from the sticky IMRC I still have.

Have you also made sure it isn't maybe your clutch slipping under heavier load?

 

[toms97cobra]

Thanks guys ..... to answer your question ... here you go..... the car is totally stock..... stock coils and wires... they are the original ones that came with the car..... fuel filter is still the original.... the car only has 34,000 miles on it.....I can see that the IMRC's could be gunked up but to put it in perspective the throttle body doesn't even have a speck of carbon in it.... ill recheck the plugs but I just put new ones in and gapped at 55ish...... and I know that the clutch isn't slipping.....

have you guys heard of gapping the plugs less .... I have read around 45 will stop the stutter at high rpm..... ill get autolites soon ..... I would love to delete the IMRC's, but don't really wana mess with it ..... if I did I think I would rather get some 2 valve heads if they have em... but the car is so pristine and perfect I can deal with the split seconds of lag ..........
now there is one thing I have noticed and that is ... it doesn't seem to do it when the car is cold....that's kinda why I was thinking that the controller is dropping or flutters at high rpm.... if it was my old 5.0 I would say its timming because that's what it feels like ... but its not......and I have also considered that this is just the way they run and there is no fixing it ...

 

[toms97cobra]

also cant I just zip tie them open .....yes I loose my low end but wont that tell me that it is ignition not the IMRC's

 

[Cowtown]

I gapped my autolites to .54 and it worked well for me. The ones in there were gapped in the mid 30's and the car ran like crap with them like that.

As for zip tying, you sure can do that, you will throw an engine code so just be aware of that. Not sure if you've thrown a code reader on it, may not be a bad idea either way if you are having a hard time chasing it down. I'd also say replace that fuel filter as a good early measure too, won't hurt and it's an easy thing to clear off the list.

 

[Tickle]

I'd start with the fuel filter myself. Cheap/easy. Change one thing at a time.

 

[toms97cobra]

Is this issue common .... Is the motor that sensitive

 

[NOVA98Cobra]

Welcome to our community. This is pretty common, and well known especially with those of us who road race and spend a lot of time in high RPMS. The IMRC's and correct spark plug gap is frequently the problem. Here is a suggestion I think is a logically way to appraoch your problem, starting with free things and going more expensive:

1st Check the free things:
1. Zip tie you IMRCs open and see if that fixes everything
2. Check your plugs for the correct gap and look to see if they are fauled up
3. Compression test (It only takes an hour and tells you a lot, you can rent the tester for free)
3. Go up to advanced auto and have your alternator tested

2nd attack the electrical system:
4. Replace plugs and gap to .54
5. Replace wires or switch to COP
6. Replace coil packs