98 V6 Wont rev over 3500 - Detailed

New cam sensor changed nothing as expected.

Hooked up vacuum gauge and it held about 23 up to about 3500rpm when the problem starts then it gradually started dropping eventually to 0.

Took it for a drive and acts the same. Runs perfect in all gears under 3500rpm. Problem occurs at same point in gear. Tested manually dropping to L then 2 then D.

At the point now of taking it to a shop to inquire about weak valve springs and get a quote on head work.

Wonder what we could get for it as is :)
 
BTTT....Any suggestions/info etc would be extremely helpful.

Right now, my dad is looking into having a engine rebuilder run diagnostics and possible do a head job. We're at our wits end with this.
 
I would *really* doubt it's valve springs. There's no way that a bunch of them would go soft and all start floating at the same RPM. It almost certainly (IMO) has to be electrical/electronic. Since it isn't load dependent, it's almost certainly not fuel flow related. It's probably not air flow related, partly for the same reason, partly because you've replaced everything that could impede air flow.

Double check the cam timing? Verify that the motor is properly at TDC #1 (timing marks are not always reliable if they're on something that might have inadvertently moved), and that the cams are properly lined up?
 
And verify ignition timing with an olde fashioned timing light? Rev it up and see if something weird happens at 3500 rpm.
 
I am as stumped as others.

With the right scanner the problem hopefully will be quite apparent.

I really want to know what the problem is.
 
Drove it again just because. Accelerates normal in D, L or 2 until about 3500.

Could the ignitor cause this without a code? Would it run so normal at lower rpm if something like the ignitor or maf was bad?
 
I once had a 280zx that i stupidly washed its engine, then it would not operate above about 2500rpm. Took it to a real mechanic and he said an electronic RPM limiter had gotten shorted out with the water. Because it didn't work, the ecm limited the rpm to that level. Not sure what is causing your issues, but if i were you I'd seek a professional mechanic I trust. I mean i can appreciate the "lets fix it ourselves" bit but at a certain point ya' gotta move on to bigger and better issues. Luck!
 
jgue467 said:
I once had a 280zx that i stupidly washed its engine, then it would not operate above about 2500rpm. Took it to a real mechanic and he said an electronic RPM limiter had gotten shorted out with the water. Because it didn't work, the ecm limited the rpm to that level. Not sure what is causing your issues, but if i were you I'd seek a professional mechanic I trust. I mean i can appreciate the "lets fix it ourselves" bit but at a certain point ya' gotta move on to bigger and better issues. Luck!
Click to expand...

I agree and we've consulted "real mechanics".
The previous owner had it at the dealership and a indy mechanic before I bought it.
Today, my Dad took it to another mechanic thats run live data scanner on it and has it now to continue testing.

MAF was finally swapped with known good, so we know thats good.

And this isnt a rev limiter issue. Problem starts roughly 3500rpm every time, but you can force it to rev to about 4k by continuing to press the throttle. The condition just worsens to the point it simply wont rev over 4k and it wont stay there, runs extremely rough the more throttle you try to give it over 3500.

I'm very familiar with rev limit from autocrossing :) and this doesnt act that way at all.

Thanks for the input though....
 
its a 98 so it only had the single cat right? and did you check whether or not more exhaust was flowing after you dropped it all? also no PO420 code yet? might be redundant but the more info the better!

and just to clarify, 3500 rpm no matter what? (i.e., driving, revving in neutral/park)

just searched this problem, one guy said to try checking grounds on the injectors, if they all don't work at 3500 rpm then something is shutting them off.

kinked fuel line causing not enough fuel for 3500 rpm? plugged/kinked/blocked hoses?

those are all the guesses I have for now. Good luck!
 
sportrunner2001 said:
its a 98 so it only had the single cat right? and did you check whether or not more exhaust was flowing after you dropped it all? also no PO420 code yet? might be redundant but the more info the better!

and just to clarify, 3500 rpm no matter what? (i.e., driving, revving in neutral/park)

just searched this problem, one guy said to try checking grounds on the injectors, if they all don't work at 3500 rpm then something is shutting them off.

kinked fuel line causing not enough fuel for 3500 rpm? plugged/kinked/blocked hoses?

those are all the guesses I have for now. Good luck!
Click to expand...


- Single cat, open exhaust where cat was dropped and same issue
- No codes ever unless forced by removing a sensor etc
- Fuel pressure is normal even when issue is occurring
- Will check grounds for injectors
- yes, 3500 no matter what, its very consistent when this occurs

- knock sensors tested and wiring harness replaced
- several vacuum lines replaced just because
- in process of testing ignitor
- in process of manually verifying timing by removing spark plug, inserting dowel and ensuring TDC by hand

What does the charcoal box on the drivers side do?
Has several electrical connections and several vacuum lines running in/out of it.
 
Igniter tested, but unsure of results given the instructions I found so we ordered a replacement for $20 from ebay.

Manually validated the timing with dowel rod in piston 1 spark plug tunnel.
Compared to both cam pulleys and everything lines up.
Crank pulley was at zero degree mark with all lined up.

If igniter doesnt do it, we dont know where else to turn. We've replaced just about everything.

Fuel pressure regulator was replaced too.

Any and all help/suggestions are greatly appreciated before we :gun4: to the truck .....
 
HelpMeFixIt said:
Igniter tested, but unsure of results given the instructions I found so we ordered a replacement for $20 from ebay.

Manually validated the timing with dowel rod in piston 1 spark plug tunnel.
Compared to both cam pulleys and everything lines up.
Crank pulley was at zero degree mark with all lined up.

If igniter doesnt do it, we dont know where else to turn. We've replaced just about everything.

Fuel pressure regulator was replaced too.

Any and all help/suggestions are greatly appreciated before we :gun4: to the truck .....
Click to expand...

Maybe the egr valve stuck open?
 
This is a LONG shot. Check the 20 amp EFI fuse. I had a 01 that had similar symptoms. Turned out one leg of the fuse was broken/burnt. Replacement solved the problem. I doubt it but check the fuse anyway.
 
Replaced ignitor and checked the recommended fuse.
We got nothing.... :(

Next thing I think will be to blow out the fuel line even just to be sure there is no clog.
 
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