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Can't Rev High Under Load?

GhostDakota

GhostDakota

NAXJA Forum User
Location
Phoenixville, PA | Bethany, DE
Recently.. my intake noise became louder and more stressed sounding. This morning I couldn't rev past 3.5k while under load. I can pop in the clutch pedal and rev and it goes up to whatever (past redline)... any idea wtf is going on here? I just put in a brand new Performance Distributors Firepower Ignition kit (Plugs, wires, cap, rotor) yesterday to see if that fixed anything ... nothing changed. I am at a loss as to what is going on... idea's?

91 XJ, 93 4.0L, 01 NV3550, 62mm TB & spacer, snorkel, etc.
 
Matthew Currie said:
Intake noise? Have you checked vacuum?
Click to expand...

Have not checked into the vacuum. Where would be my first step doing this? I've looked at the vacuum lines for cracks or unplugged ones, all looks good except 2 I have found. One small hard tube on the passenger side that I just taped off. One on the driver side (big rubber tube.. I think it's a vent).


IGOCOMMANDO said:
Clogged Cat?
Click to expand...

That's possible too but wouldn't I be smelling "rotten eggs"? I think you may be onto something though. I do have a whole new exhaust system I can throw on there too.
 
GhostDakota said:
That's possible too but wouldn't I be smelling "rotten eggs"?
Click to expand...

Not necessarily. I've had two bat cats on two different Jeeps. The first, on my 1991 4.0 Cherokee was the worst; the symptoms were a muffled sounding exhaust, serious strain in order to get up any speed, and a slight hiss. The second, on my 1993 5.2 Grand Cherokee wasn't as bad; the symptoms were simply a rattle every now and then, and some loss to gas milage.

Hope you track it down!

:)
 
XJPhoenix said:
Not necessarily. I've had two bat cats on two different Jeeps. The first, on my 1991 4.0 Cherokee was the worst; the symptoms were a muffled sounding exhaust, serious strain in order to get up any speed, and a slight hiss. The second, on my 1993 5.2 Grand Cherokee wasn't as bad; the symptoms were simply a rattle every now and then, and some loss to gas milage.

Hope you track it down!

:)
Click to expand...

Understood. I'll try to get the new cat and back exhaust I have put on at some point. It was 80 Monday and now it's supposed to snow tonight... how ridiculous is that?! Thanks for the help!
 
XJPhoenix said:
Ugh, I hear you!

We've had snow now for the past two days, and I'm already plenty sick of it. On the plus side, I hope it will kill the mosquitos!

:)
Click to expand...

That stinks man.. I don't want ANY accumulation.. I want 80* and I will be happy! I just hate the bugs.. like you do... damn mosquitoes and bees!

I really hope it's the cat.. I am going to cut everything off today... and then get my whole new system put on on Sunday or Monday next week. I'll know immediately if it is the cat or not.. damn this is going to be loud.. I don't want to deal with this ... I have a 1 1/2 hour drive to a birthday dinner, then a 2 hour drive to my shore house and a 2 hour drive back... all highway... that's going to be lovely.. I need to find some ear plugs for myself and my lady friend lol.
 
Actually, I'd avoid driving it without exhaust. It can be bad for the motor, and the engine just won't make good power. For some reason, the 4.0 needs a certain amount of backpressure; the more technically inclined will be able to explain it. All I know is that some folks here have actually had some exhaust valve damage from running without enough exhaust pipe.

And yes, it'd be REALLY loud. Kinda cool though if it was for short rides...
 
XJPhoenix said:
.....the 4.0 needs a certain amount of backpressure
Click to expand...

Engines DON'T need backpressure. It's excessive backpressure due to a clogged cat that's causing the problems that GhostDakota described.
What engines need is the right amount of exhaust scavenging so that only waste gases, and not fresh air/fuel mixture, enter the exhaust manifold. That's where selecting the correct pipe length and diameter becomes important. "Backpressure" keeps waste exhaust gases inside the cylinders, and these dilute the next fresh air/fuel mixture charge causing a reduction in performance.
 
GhostDakota said:
Have not checked into the vacuum. Where would be my first step doing this? I've looked at the vacuum lines for cracks or unplugged ones, all looks good except 2 I have found. One small hard tube on the passenger side that I just taped off. One on the driver side (big rubber tube.. I think it's a vent).




That's possible too but wouldn't I be smelling "rotten eggs"? I think you may be onto something though. I do have a whole new exhaust system I can throw on there too.
Click to expand...

I'm not sure what tubes you've found there, but it sounds as if you might want to look further at vacuum lines along with checking the cat. Vacuum gauges are cheap and very handy for diagnosis. Most come with basic instructions on what to look for, and how to read symptoms.
 
Dr. Dyno said:
Engines DON'T need backpressure. It's excessive backpressure due to a clogged cat that's causing the problems that GhostDakota described.

What engines need is the right amount of exhaust scavenging so that only waste gases, and not fresh air/fuel mixture, enter the exhaust manifold. That's where selecting the correct pipe length and diameter becomes important. "Backpressure" keeps waste exhaust gases inside the cylinders, and these dilute the next fresh air/fuel mixture charge causing a reduction in performance.
Click to expand...

Gotcha. Sort of... :)

All I really understand (and can wrap my bitty little brain around) is that with wide open pipes the 4.0 just doesn't seem to run as well. Too much restriction = bad, and too little restriction = bad as well.

Was I right about the open pipe leading to badness for the motor? I've never done it myself; just read more than a few folks who've damaged their exhaust valve by having just a header, or very little exhaust pipe.

Ghost Dakota: Another thing to consider is the possibility of being pulled over while having no exhaust to speak of. If a copper heard you tear by in your Jeep with no exhaust to speak of, you might very well wind up with some nasty fines.

:)
 
I pulled the exhaust on Friday.. drove all weekend and today with it like that. It's loud, but not much louder than my Dakota. It's bearable. I have power. It's crazy power.. I've never had this before in a Jeep.

Anyway, I should have the new system thrown on this week and the Jeep will most likely be gone by next weekend :( There goes my 2 year NAXJA membership... blah. Oh well!

I'll be checking into the vacuum too before it goes. Thanks for all the help everyone!
 
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