Noticed this issue when I was driving cross country, the longer I would drive the more throttle it would take to maintain whatever speed I was cruising at. Turn it off for a few minutes and it the power comes back, only to slowly lose it again.

I've also recently noticed a whining noise above 3k rpm, probably not related but you never know.

I'm wondering if it could be fuel system related, but I really have no idea.

My searches haven't turned anything up for minor loss of power, plenty of people having issues with stumbling engines etc.

Any ideas?

Its a '99 4.0 aw4.

 

Might need to do some noggin pokin.

When was last time axle and TC level checked? God forbid even changed?
If low or worn out things could heat up and increase friction.
Didn't say how long it had to "cool down".

 

I had a similar issue. Turned out to be seized calipers.

 

Don't think my calipers are seized but I'll double check.

Front hpd30 and rear 8.8 got new fluid about 4k miles ago.
Tcase fluid is nice and red, changed I think 15k miles ago. Transmission fluid is also nice and red, changed about 4k miles ago.

Cool down period is like 5 minutes.

 

catalytic converter?

 

My thoughts.

CRUISER’S VACUUM TEST FOR EXHAUST RESTRICTION

Your vacuum gauge should come with an instruction booklet outlining the procedure. Hook the vacuum gauge up to a source on the intake manifold. Start the engine and note the vacuum reading. Usually 17 to 21 inches of vacuum. Throttle the engine up to 2,000 to 2,500 RPM for 20 seconds or so and the vacuum reading should stabilize to the same reading you got at idle. Let the throttle snap shut. The vacuum reading should shoot up about 5 inches of vacuum higher for a second and then come quickly down to the original reading. If the vacuum reading stays high and comes down slowly with jerky needle movements, you have an exhaust restriction.

 

What do the RPMS do when this is happening? You say it takes more throttle pressure to maintain speed. Do the rpms rise as well? For instance normal cruise might be 2200@60 but instead it goes to like 3000@60?

The only reason I ask is because I had a ZJ once that would do this and it ended up being the filter in the trans. It would get clogged up after 10-15 min of driving, I'd pull over let it cool and would be good for a little bit again. I ended up replacing the trans.

The fuel issue and Cat issue seem much more likely though. DO you have a small scan tool? You could monitor long and short term fuel trims while this is happening and gain a very solid view about whats going on.

 

Hmm catalytic converter def a possibility. I seem to recall reading that plugged exhaust can cause a whining noise? That might explain the whine I've noticed recently. Thanks the testing instructions cruiser!

Assuming I want something like this for said test: http://www.napaonline.com/Catalog/Ca...715_0006396217 ?

tlang, rpm and speed are linked, no weird behavior that I've noticed.

I actually have a laptop based scanner and ScanXL, which has quite a lot of functionality. I haven't messed around with watching fuel trim but I don't think it'll be too hard to do a data capture next time I drive the jeep.

 

Finally got around to the exhaust restriction test. Not sure what to make of it. Video attached. Sits at 15" at idle, stabilizes at 18 or so at 2k RPM, and I'm not sure if this drop constitutes as slow or not.


Any thoughts? Still need to monitor fuel trim, may do that tonight as well.

Edit: I monitored fuel trim and both long and short term were jumping all over the place (would be at say 7%, jump to 50% and right back down to 0 etc etc) with the engine at idle and at a constant throttle position above idle.

 

Once I have a similar issue, and turned out being the brakes, somebody installed a new master cylinder and have the rod that pushes the piston a bit too long, that was enough for the pressure on the lines to get increased as the temperature of the brakes increased.

After several mechanics being consulted we looked at the brakes, shortened the rod and problem fixed

 

Once it stabilizes at 2000 rPM, to what it was at idle, let the throttle Snap shut not go down slowly.

 

That's the weird thing, it never stabilizes at the same pressure as at idle. Reads ~3 psi higher. I just re-ran the test tonight and got 13 at idle and 16 at 2k rpm. Tonight was cold, last time was hot.

New development though, my intake/exhaust manifold gasket is giving up, that probably invalidates the vacuum testing. Guessing its from high underhood temps associated with with the wheeling I've been doing in moab recently. Also the POS APN header I'm running..

 

Concentrate on what happens when the throttle SNAPS shut.