98 V6 Wont rev over 3500 - Detailed

[Troybylt]

Is the truck an auto trans?

It's probably a long shot but I had a car that "pink milkshaked" and before thing's went to shit with over heating and what not it would do that, only rev to about 3k.

Have you diagnosed anything with the transmission? Even if it's not currently milkshaked the tranny may be fubared?

 

[Kenworth]

The box on left fender is vapor recovery ,I have used one here,but don't think that could be issue

 

[Kenworth]

Bump any luck finding a solution

 

[twa318]

off topic, but I don't think I've ever seen my V6 hit 3500 rpm (4.10 gears).

bump for visibility.

 

[Kenworth]

Any luck on this ?

 

[HelpMeFixIt]

Any luck on this ?

Nope...

We've replaced the ignitor, blown out the fuel lines all the way back to the tank and dropped the tank again and emptied it.

New Fuel Pressure Regulator, swapped MAF for known good....
Replaced some cracked vacuum lines, tested knock sensors and replaced plug/cable.

We want to scan it with a live scanner, but having trouble getting someone over. We'd rather pay someone cash to come over than pay a shop possibly way more for potentially nothing.

 

[HelpMeFixIt]

What would I be looking for if this is a tranny issue?
I can drive it fine, just not over that RPM range.

If it does it in neutral and park, I dont see how a fubar'd tranny could cause this unless its a sensor or something.

 

[HelpMeFixIt]

Rebuilt heads installed and same issue - almost sounds like limp mode at 3500 but if you've read this thread, you'll see absolutely everything we can think of has been replaced.

No idea about the tranny sensors, where they are or how to test them.....

Please help.........

 

[DionJ]

I'd guess electrical. Sometimes when alternator bearings are failing there is enough wobble at higher revs to float the brushes. Maybe even before alternator check all ground wires.

 

[Diezel]

Couple of concerns and questions.

Was there a compression test done, If so, what was the numbers?
Exhaust was checked, but no mention of intake tract.... Clean?
Pop the valve covers, and inspect the valves. The 3.4L engine is a quad cam, with 2 belt driven cams, and 2 gear driven. Check the alignment of the cams in relation to each other.

 

[St Runner]

If it is always at the same exact rpm that the problem occurs it is electrical. It seems as though you have checked all the sensors so the problem could be a short or poor connection in the wiring. Needle in a haystack to find so the easiest way is to replace the engine wiring harness. Takes about three hours and you probably won't need a upper manifold gasket since you just did the heads. Should be able to find a used harness for cheap

 

[jhfleet]

I'd guess electrical. Sometimes when alternator bearings are failing there is enough wobble at higher revs to float the brushes. Maybe even before alternator check all ground wires.

This is a serious question because this is a new theory to me: If the battery is charged enough to start the engine, how can a bad alternator cause rough running and limit the maximum rpm of the engine?

 

[DionJ]

This is a serious question because this is a new theory to me: If the battery is charged enough to start the engine, how can a bad alternator cause rough running and limit the maximum rpm of the engine?

I would have never believed it myself, but several Spec Miata race cars have had this issue recently, yes, it's at nearly twice that rpm, but identical symptoms, and an alternator solved it. Not sure it applies in this case, at these rpm, but it's real..

 

[HelpMeFixIt]

The first question anyone is going to ask is "have you checked the codes?"

This has been addressed SEVERAL times - There are no codes and never has been unless forced by disconnecting a sensor etc. Left alone, no codes are generated.

 

[HelpMeFixIt]

Couple of concerns and questions.

Was there a compression test done, If so, what was the numbers?
Exhaust was checked, but no mention of intake tract.... Clean?
Pop the valve covers, and inspect the valves. The 3.4L engine is a quad cam, with 2 belt driven cams, and 2 gear driven. Check the alignment of the cams in relation to each other.

Compression test was done, dont have #'s now, but they were all the same.
Intake has been checked, cleaned etc....

Heads were just fully rebuilt, valves are new, springs etc.
Cams/timing have been checked and set multiple times.

 

[HelpMeFixIt]

I would have never believed it myself, but several Spec Miata race cars have had this issue recently, yes, it's at nearly twice that rpm, but identical symptoms, and an alternator solved it. Not sure it applies in this case, at these rpm, but it's real..

We'll check it out, thank you for the suggestion.

 

[jhfleet]

I would have never believed it myself, but several Spec Miata race cars have had this issue recently, yes, it's at nearly twice that rpm, but identical symptoms, and an alternator solved it. Not sure it applies in this case, at these rpm, but it's real..

Thanks!

 

[cashmoney]

On the corvette forum I'm on a guy with a Z06 was having a similar issue in his car. Ran great below 4500rpm but above that it chugged, bogged, shuddered, wouldn't rev. Ran it on the dyno, multiple scans, multiple parts, nothing jumped out. Replaced the alternator and the problem was solved instantly. The pulley was wobbling enough to effect the brushes and was tripping the knock sensor in his car.

 

[HelpMeFixIt]

Got it... will definitely check it out.
If thats it, my dad will be pissed as will the previous owner... LOL

 

[SnowLizard]

alt is a good bet,

if not, possibly a failed TQ converter, I've had one fail in my 65 mustang i6, one day it just couldn't rev above 3200 without SCARY engine vibes (carb so no limp mode), I went through everything I could, even rebuilt engine & trans, it wasn't until someone suggested the TC that I replaced it... and it was gone after that. and I thought I was saving buck re-using it when it was that all along.:frusty: