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Unread 01-01-2012, 11:38 AM   #1
oskar
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1988 YJ Wrangler 
 
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Loss of power when warming up

I've got a new set of problems on my Wrangler. Back story on this, I bought it a couple of months ago, have been driving it issue-free until last night. It's the 4 cylinder with a manual transmission.

Last night, the Jeep fired right up, and I started heading down the road. All of a sudden it felt like a huge breeze hit and I started losing power. I thought 'must be a Jeep thing and a windy night', but it only got worse. I was almost to my destination, so I limped it there limited on power. It felt like I was towing a trailer uphill, and was being held back. When waiting to turn left, it stalled and I had to restart it and give it some gas to get moving.

I thought my fuel pump might be going bad.

This morning, I started the Jeep and it fired right up like it always had...weird. I started driving and it felt normal again, and I got a mile or so down the road until it started happening again. This time I noticed that none of the problems happened until the coolant temp needled moved from the cold resting position. I realize the 4 cylinder is pretty underpowered, but this is worse than normal!

Now I believe it's sensor-related. Could also be fuel-related, but it seems pretty odd that as soon as it acted up the needle had just moved.

I tried to order the coolant temp sensor on the intake manifold a few weeks back, but I ended up getting one with the wrong connector on it. If anyone thinks that's the problem, does anyone have the part number (again, 4 cylinder manual trans)?

I work at an auto parts store and will rent the fuel pressure tester just to rule fuel out, and since buying the Jeep I've replaced plugs/wires/cap/rotor, so those should all be good. Even after getting it home today, the engine sounded healthy at idle and had normal throttle response when stationary. Seems like it's only happening with a load under it.

For reference, ambient temps have been high 30's-low 40's last night and today, so I don't think I'm experiencing vapor-lock.


Help! the snow is coming and I want to be ready

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Unread 01-01-2012, 08:04 PM   #2
moonshinefuel
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I don't mind throwing out guesses here.
It could be that it is simply building up back pressure, and the catalytic converter is clogging up.
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Unread 01-01-2012, 08:12 PM   #3
oldtime_ironman
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Its likely either the coolant temp sensor or the O2 sensor. Reason being the ECU doesn't read the O2 at all until it senses that the engine is up to a certain temp. The temp sensor should show about 185 ohms at 200 deg F. The O2 should have around half a volt on the sense wire to ground (with a good multimeter). I would actually bet on the O2 sensor first. Hope this helps!
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Unread 01-01-2012, 08:13 PM   #4
oskar
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Quote:
Originally Posted by moonshinefuel View Post
I don't mind throwing out guesses here.
It could be that it is simply building up back pressure, and the catalytic converter is clogging up.
I feel dumb now, but I think it was just the wind; we've had 40mph winds over the past two days. I was I'm 5th gear at about 55mph which is probably too tall, and the jeep never backfired or anything (probably the right fuel mixture). I bought a new fuel filter and am going to seafoam the engine, hopefully it will restore some lost power over the past 180,000 miles. After I fix the rear of the frame, I should be in business for the winter


Thanks for the help, I'll report back if it gets any worse
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Unread 01-01-2012, 08:17 PM   #5
oskar
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Quote:
Originally Posted by oldtime_ironman View Post
Its likely either the coolant temp sensor or the O2 sensor. Reason being the ECU doesn't read the O2 at all until it senses that the engine is up to a certain temp. The temp sensor should show about 185 ohms at 200 deg F. The O2 should have around half a volt on the sense wire to ground (with a good multimeter). I would actually bet on the O2 sensor first. Hope this helps!
.
I was planning in replacing the o2 sensor and the coolant temp at some point anyway, I read the guide on modifying the 98 sensor to work for my yj, but is there an easier way?
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Unread 01-01-2012, 08:35 PM   #6
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I would do a full tuneup, new cap, rotor, plug wires, plugs, fuel filter, air filter, oil change, engine flush, fuel injector cleaner, replace all of your sensors and see where you are at.
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Unread 01-01-2012, 08:56 PM   #7
oldtime_ironman
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Quote:
Originally Posted by oskar View Post
I was planning in replacing the o2 sensor and the coolant temp at some point anyway, I read the guide on modifying the 98 sensor to work for my yj, but is there an easier way?
For the coolant temp sensor, it has been found lately that you're way better off to get the actual dealer part. Most of the common aftermarket ones from places like Autozone, etc. are way out of spec even brand new.

For the O2 sensor, go 93 and earlier. Be aware also the wiring is different between the 4-cyl and the 6-cyl -- ISTR the 4-cyl has a 3-wire connector, the 6-cyl has 4 wires. Sometimes the O2 connectors give people a pain when the wire breaks inside the insulation - it doesn't hurt to check continuity there, and make sure the harness is tied up out of the way.
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Last edited by oldtime_ironman; 01-01-2012 at 08:58 PM.. Reason: cts
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Unread 01-01-2012, 09:06 PM   #8
NonRubicon
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Quote:
Originally Posted by moonshinefuel View Post
...It could be that it is simply building up back pressure, and the catalytic converter is clogging up.
This is what I was thinking. I've experienced intermittent power loss (like hitting a rev limiter) due to a broken up cat converter. Chunks of the cat were occasionally blocking the cat outlet. Started up just fine, rev in neutral just fine, but go about half a mile and then it would feel like power was being sapped. Eventually a big enough chunk became permanently wedged in the outlet.
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