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Possible little or no compression?

X

XJFever

NAXJA Forum User
Location
Anchorage, Alaska
My friend brought their 1990 Jeep 4.0 to Midas, who told them about the compression. Not sure if it is thought. It idles fine, just when she gets up to about 35-45 mph and steps on the gas, it acts like it wants to speed up, then immediately the RPM's drop to about 1500. It feels like a spark plug problem.

When I checked the spark plugs, I noticed the one in cylinder 3 was cracked. After I replaced it, it still had that problem. Thing is, I have no compression tester to test it myself. I started up the jeep, and put in neutral and revved it up and it got pretty high with no problems. Only problems around the speed I mentioned. Does it sound like it could be a compression problem?
 
Need more information. What did Midas base their "compression problem" diagnosis on? With the symptoms you describe, diagnosing a compression issue off the cuff seems very suspect. Did they actually TAKE compression readings??? If so, post the exact numbers here for comment.

I'd start by testing and adjusting the throttle position sensor; it's adjustable on a 1990. Maybe somebody with more RENIX knowledge will chime in with some more specific ideas.
 
Last edited:
K, a little misleading there.

I called them up and they told me it wasn't the compression but that they heard a lower engine knock and that it was time for a new engine. Thats why it would lose power going up hills around that speed. Sound right?
 
Get another opinion on the lower engine knock that Midas heard from a technician from an ASE shop. Noises can be tough to diagnose properly.

Testing fuel pressure at the schrader valve on the rail and testing the TPS would be what I would do if it were my vehicle. A new fuel pump doesn't mean you couldn't have a fuel delivery problem of some sort. Testing fuel pressure takes only a few minutes.
 
A lower engine knock would not affect performance. At least not until the engine comes apart. But, I'm not agreeing that there is a knock or not. Just saying that a knock is a different issue than a performance problem.
 
if every xj with a knock needed a new engine, we all would need a new engine.

since when does a knock equal new engine.WTF
 
XJFever said:
Where would I start. The midas guy said it wasn't having problems with compression. Just the fact that there was a knock was why it was loosing pressure.
Click to expand...

The midas guy is an idiot. Take your Jeep elsewhere and never go back! You could have a knock so bad that scares the crap out of the driver next to you and it still wouldn't cause your symptoms. I agree with the previous posters: test the TPS and fuel pressure.
 
BTW, how much gas was in the tank when going uphill?
 
Given the limited info, my best guess would be throttle position sensor.

A second guess might be a plugged cat.
 
XJFever said:
My friend brought their 1990 Jeep 4.0 to Midas, who told them about the compression. Not sure if it is thought. It idles fine, just when she gets up to about 35-45 mph and steps on the gas, it acts like it wants to speed up, then immediately the RPM's drop to about 1500. It feels like a spark plug problem.
Click to expand...

Is it actually losing power, or simply upshifting into 4th gear when it shouldn't? This might be as simple as a bad TPS sensor. Does it not do this or seem to have plenty of power if you have the shifter in (3)?
 
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