4000RPM's = Brick Wall???

[mcaracter]

This is in regards to my 1991 Daytona Shelby. I've already checked the cam timing and the ignition timing and both are spot on, however, when I reach 4000 to 4500 RPM's (it varies and I'm not sure why) it's like I'm hitting a brick wall. And no, it's not overboost shutdown. I've been through that before and what I'm experiencing feels like the engine hitting an invisible ceiling of power not a sudden cutout. This doesn't make any sense to me.

Any suggestions? A friend of mine suggested a new computer, but without some solid evidence I'm hesitant to spend the cash.

-Walt

[turbo1168]

When was the last time you changed the fuel filter?

[mcaracter]

Quote:

Originally Posted by turbo1168 When was the last time you changed the fuel filter?

That's an excellent question, and I think I see where you're going with that. However, the filter was replaced along with the fuel pump, and sending unit back in February.

[turbo1168]

You might want to check your fuel pressure anyways. Could be the fuel requirement is exceeding the fuel delivery.

[xrattiracer]

what kinda shape is the exhaust in? (plugged cat?).

[speeduphoria]

the main question is: is it at all times or only in boost?

if only in boost then check your spark, meaning plug gaps/coil/wires. Also could be cat or fuel issue

if all the time then i suspect cam timing is off a tooth

[Fiver ShelbyZ]

Just to add food for thought: I had an intermediate shaft what was worn and finally gave up, mine was doing the same thing, brick wall at about 4k RPM. Every time we checked timing it was spot on, except when it finally jumped teeth. All symptoms pointed to over boost or fuel related issues. (But it never over boosted)

[mcaracter]

Quote:

Originally Posted by turbo1168 You might want to check your fuel pressure anyways. Could be the fuel requirement is exceeding the fuel delivery.

That's a valid concern. Unfortunately I won't have any answers on that until I can track down a guage.

Quote:

Originally Posted by xrattiracer what kinda shape is the exhaust in? (plugged cat?).

The exhaust is actually pretty free flowing. I have a 2.5 inch mandrel bent exhaust with no cat and a very low restriction race muffler with the pipe exiting out the side just in front of the rear tire so I suspect no flow issues there.

Quote:

Originally Posted by speeduphoria the main question is: is it at all times or only in boost? if only in boost then check your spark, meaning plug gaps/coil/wires. Also could be cat or fuel issue if all the time then i suspect cam timing is off a tooth

I'll have to do some diagnostic driving to find the answer to that one. To be honest, I usually shift around three thousand RPM's for gas milage.

Quote:

Originally Posted by Fiver ShelbyZ Just to add food for thought: I had an intermediate shaft what was worn and finally gave up, mine was doing the same thing, brick wall at about 4k RPM. Every time we checked timing it was spot on, except when it finally jumped teeth. All symptoms pointed to over boost or fuel related issues. (But it never over boosted)

How does one go about testing for a worn intermediate shaft?

Thanks for all the input guys. I would like to add some extra input to the situation to see if it helps This engine has been upgraded with the afore mentioned exhaust as well as a garrett instead of the mitsu. I also have a ported head that a friend of mine had alot of success with on his LeBaron. However, on his car he was running a 2.2 with a mitsu, not a 2.5 with a garrett. Is it possible my head is having a negative influence??

p.s. sorry for the long post

[turbokid]

Sounds like timing 2 me
I have had the same problem

[mcaracter]

Quote:

Originally Posted by speeduphoria the main question is: is it at all times or only in boost? if only in boost then check your spark, meaning plug gaps/coil/wires. Also could be cat or fuel issue if all the time then i suspect cam timing is off a tooth

Actually, I can't get the car up to 4000 rpm's without building some boost. So I guess the answer is yes, it's happening under boost. It lookes like I have some investigating to do...

[turbokid]

could also be the hep!
Does it shudder under load?
Take a look at the HEP make sure the wires are not bent outa shape, make sure the disk is not warped etc.
Then triple check your timing =)

[speeduphoria]

Quote:

Originally Posted by mcaracter Actually, I can't get the car up to 4000 rpm's without building some boost. So I guess the answer is yes, it's happening under boost. It lookes like I have some investigating to do...

I forgot to ask another, whats your idle vac? that should rule out cam timing

[mcaracter]

Quote:

Originally Posted by speeduphoria I forgot to ask another, whats your idle vac? that should rule out cam timing

She starts a little rough, but once she's idling she pulls a little over 15 inches of vacuum.

[1991shelby]

That seems low to me. I am pulling 22-23 at idle. Mine did not change much even with my injector o-rings and a vacuum line leaking. My cam timing was a tooth off also and I was still around 21.
My car felt the same (stopped making any power at 4500 rpms). I could of held it there for ever and never gained any speed.
I fixed the leaks but most of all fixed the cam timing and instant power.

[mcaracter]

Quote:

Originally Posted by 1991shelby That seems low to me. I am pulling 22-23 at idle. Mine did not change much even with my injector o-rings and a vacuum line leaking. My cam timing was a tooth off also and I was still around 21. My car felt the same (stopped making any power at 4500 rpms). I could of held it there for ever and never gained any speed. I fixed the leaks but most of all fixed the cam timing and instant power.

Well, then tomorrow after work I think I will double, tripple, and quadrouple check my cam timing. I think I'm also going to remove my Grainger valve and return to computer controlled boost and see if that makes a difference.