Stumped! 1989 4.2 loses power at 2000+ rpm - JeepForum.com
 
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post #1 of 13 Old 02-28-2014, 09:14 AM Thread Starter
rexracer530
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Stumped! 1989 4.2 loses power at 2000+ rpm

1989 carter carbed 4.2. I searched around a bit before posting but these symptoms seem to match various things (carb, timing, tps, clogged cat) and I don't know where to start first. The Jeep runs normal below 2000rpm / ~45 mph but loses power at speeds above that and the RPMs drop down to around 1000rpm suddenly.

I had lack of power issues before (over a year ago) and at that time it was the carb dribbling fuel. Cleaning the carb fixed it and i drove with no problems for a long time. So thinkg it might be the same thing I checked that again and it seemed to be spraying fuel normally into the bowl and operating normally.

Has anybody had these loss of power (accompanied by rpm drop) issues before? Some searches made it sound likes clogged cat and other sound like it might be TPS. I don't want to start just randomly throwing money at it... Any ideas where to begin? It's carbed so no engine codes to help me diagnose it.


Last edited by rexracer530; 02-28-2014 at 09:19 AM. Reason: Phone fat fingers
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post #2 of 13 Old 02-28-2014, 09:29 AM
Rproject
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Have you checked for vacuum leaks? Mine bogs down hard at higher speeds due to a vacuum leak associated with a blown manifold gasket. When I put my hand over the carb, to choke it out when warm, I get virtually no idle change. That's how we diagnosed it as the manifold gasket. It's a quick diagnosis check that doesn't require parts.

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post #3 of 13 Old 02-28-2014, 09:48 AM
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Mine would stumble and lose power over about 2000 RPM. I swapped in an HEI distributor and it made an immediate difference. Chances are the advance in the old distributor was not working properly but it's easier to replace than fix. I did a Howell kit not long after and between the two it's like installing 2 extra cylinders

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post #4 of 13 Old 02-28-2014, 10:55 AM
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Mine falls on its face at 3k rpm and sounds like it misses , Im sure mine is cause by a worn distributor. One day I'll switch to HEI

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post #5 of 13 Old 02-28-2014, 11:16 AM
dancytron
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Have you done a regular tuneup yet? You can do it with better Ford parts. https://www.jeepforum.com/forum/f8/i-...nitive-669495/

Like they said above, check for vacuum leaks.

Have you done the Nutter yet? If not, it could be a sensor thing. http://home.sprynet.com/~dale02/list.htm

Your engine doesn't have a TPS.
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post #6 of 13 Old 02-28-2014, 11:25 AM
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Check for a weak fuel pump.


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post #7 of 13 Old 02-28-2014, 11:44 AM
bpounds
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Quote:
Originally Posted by jbolty View Post
Mine would stumble and lose power over about 2000 RPM. I swapped in an HEI distributor and it made an immediate difference. Chances are the advance in the old distributor was not working properly but it's easier to replace than fix. I did a Howell kit not long after and between the two it's like installing 2 extra cylinders
Were there any smog issues with an A/M distributor/ignition?

I think my vacuum advance is a little sticky and the distributor bearings a little sloppy. Loaded OEM distributors are pretty cheap, but thinking maybe I should upgrade instead. Gotta pass smog though.

Bill

1988 Wrangler,4.2L I-6, Howell EFI JP-1,CRT HEI Ign,AX15 ext slave conversion, Zone 2" lift + 1" BL + 1/2" booms, BFG 31 x 10.5, Bestop.
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post #8 of 13 Old 02-28-2014, 12:54 PM Thread Starter
rexracer530
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I did do a tuneup last summer. I think I've put less that 1000 miles on since then. I haven't nuttered it yet mostly because I'm a fan of not modding from stock if its working. I might do it now as I suspect it would work better than the stock setup.

I did spray some carb cleaner around the carb when running to see if RPMs changed but they didn't. Im not sure i knew where to spray ot though. Would it be better to put my hand over the carb throat whe it's warmed up to find out? I too have wondered if its some kind of vacuum advance problem.
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post #9 of 13 Old 02-28-2014, 02:05 PM
jbolty
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Quote:
Originally Posted by bpounds View Post
Were there any smog issues with an A/M distributor/ignition?

I think my vacuum advance is a little sticky and the distributor bearings a little sloppy. Loaded OEM distributors are pretty cheap, but thinking maybe I should upgrade instead. Gotta pass smog though.
I did the howell all legal but to be honest it never occurred to me that there might be a question about swapping the distributor. Smogged several times since and no one has ever asked about it.

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post #10 of 13 Old 03-01-2014, 09:24 AM Thread Starter
rexracer530
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Ok so I warmed it up today and then put my hand over the carb throat. The engine died. I started it up again and tried checking vacuum hose connections, especially at distributor -- didn't see anything that was recognizable as leaking although some of the vacuum hoses could probably do with replacement. Took it for a spin around the block and couldn't reproduce the problem like before. I was going about 50-55mph on the streets around my house and was over 2k rpm so now I'm even more stumped. Now I have more questions - and I still want to track down exactly what it was so I don't end up stranded somewhere later.

Was there a loose connection somewhere that I fixed by checking the vacuum hoses?
Was the problem related to engine temperature?
Was the catalytic converter / exhaust clogged and me choking out the carb caused it to unclog?
Some kind of mysterious carb problem -- maybe a jet clogged that somehow dislodged?

Any ideas guys?
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post #11 of 13 Old 03-01-2014, 09:32 AM
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I had an ECM act up like that. Started about 2250 RPM & was hit or miss....would disappear for months Worse when it was warm out. Finally started creeping down til it was kicking out at 1500, every shift.


Did the Nutter by pass & it will rev to the limit now. Never had a problem again.

probably out breaking stuff.
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post #12 of 13 Old 03-01-2014, 10:05 AM
dancytron
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If I had to make a wild guess, I'd say the knock sensor. It is a little microphone that listens for spark knock and then retards the timing when it hears it. If it retards the timing when it isn't supposed to, it could be the problem.

http://home.sprynet.com/~dale02/list.htm or do the Nutter.
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post #13 of 13 Old 03-02-2014, 08:47 AM
Dwarf
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It's embarrassing to admit - but I had a very similar issue. It's embarrassing b/c I would open the hood, check all manner of vacuum, timing, ignition, etc. everything looked solid, put the air intake back on and go drive it around, only for it to stumble under load again. In a fit of frustration, I finally drove it with th intake off and...viola! The vacuum actuators in my intake horn had frozen. Try driving without anything restricting your air and see what you get.
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