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More TPS....or something else?

Tom R.

NAXJA Forum User
Location
Utah
Seems people are having a lot of TPS issues lately, so I'm throwing this out there.

I have an intermittent power loss problem that I and several shops have been unable to diagnose. The power loss is constant during the cold months and intermittent during the summer months. Mileage went down from about 17-18 to 13-14 miles to the gallon, and pulling small hills on the highway can be a problem. It's "seat-of-the-pants" noticeable.

All sensors except for the TPS and MAT (on intake) have been replaced. When I checked the TPS voltage using my cheap DMM it was just a hair out of tolerance (IIRC, it read ~190 mV when throttle fully closed). It read good with throttle fully open. I wonder if the TPS could even cause a power loss problem.

Three shops, including a dealership, watched all the sensors function while hooked up to an analyzer/computer and said everything was operating within tolerances. My emissions test came out great, so I can assume, at least to some degree, that compression is good.

I'm open to other ideas to check for this problem. What I've done: new wires, plugs, cap & rotor, fuel filter, fluids changed, vac lines replaced, rebuilt head, welded up exh. manifold cracks, checked fuel pressure, cat replaced, and a few other things I can't think of. Currently no codes flagged.

If your advice leads to a fix, I will buy you a beer, soda, or whatever you drink. :)

Thanks
 
I had a similar problem on my Jeep. It would hesitate and stall at times. It would only happen when it was cold and in stop and go driving. It wound up being the coolant temp sensor. What was happening was the temp sensor would show the engine temp to be around -5* to around 10* until it warmed up to close to operating temp. The low temp reading would cause the computer to run the engine excessively rich and actually flood out the engine causing my hesitation and intermittent stall condition. Changed the sensor and all was well. The only way I found it was to drive the Jeep with the scanner attached and monitor the sensor values as the problem was happening. If your problem is intermittent, then it really is hard to diagnose what is going on. Maybe let the shop working on it drive it home or around town for a few days with the scanner hooked up and see if it will act up.

Bryan
 
Thanks Bryan. My XJ doesn't hesitate or stall, and I replaced the coolant temp sensor when I overhauled the cooling system.

Using the scanner while driving around sounds like a good idea.

I forgot to mention that when the power loss problem goes away, the engine runs a little smoother and noticeably quieter. When I say "smoother" this isn't to mean that during the power loss the RPM fluctuates at idle or shows any abnormalities during acceleration. I often keep my eye on the tach, especially at idle, and see no discernable changes.
 
Dang Tom, seems like you've replaced every sensor on the engine at least once and have practically rebuilt the engine. Perhaps a tranny issue?
 
I had thought about the tranny, but it seems to be an engine issue because of the way it runs & the noise in makes. Sometimes I know the power back just from starting it up......the engine is quieter.

I thought about the muffler being clogged, although that too is unlikely since the Flowmaster is fairly hollow inside. And when I had the cat replaced the first time I don't recall it being that bad, as in having any broken pieces inside.
 
jneary said:
have you ever been able to check fuel pressure when it acts up. the fuel pump could be going bad.
I only did the leak down test and fuel regulator pressure test. Both checked good. The fuel pump has always worked (hear it when keyed before start).

This is one of the first things I thought about, but November will be the two year anniversary of this problem. I'm not sure if a pump would take this long to suffer before it dies. What's your experience with them?
 
Re: Engine Power Loss

Wow, it's been two years since this thread was active. The past year I was stationed on a remote tour overseas and the year prior to that I drove my car and rarely used the XJ.

So, now I'm back and want to tackle this power loss problem. And since we have many new faces here at NAXJA I'm hoping someone can help me figure it out. Please see above posts for details. Thanks for your help.
 
So you've replaces the TPS, have you replaced the fuel filter? The only other thing I can think of is the MAP sensor on your firewall. I had intermitten issues with mine, until I changed MAP sensors and threw on an adjustable MAP sensor input. Made a night and day difference.
 
What about the CPS sensor how old is it and have the wires melted to the manifold yet? ( Mine did 2 times untill I came up with something different and Ive only had my 88 about a month now)
 
Yes, I replaced the TPS, CPS and MAP sensors, along with the fuel filter. I had another thread in OEM that had more detailed info on my troubleshooting history, but for some reason I can't find it.

Every sensor has been replaced and all tune up components replaced. While it easily passed the emissions test, the exhaust does have somewhat of a sulfer smell, despite the fact that the cat was replaced twice.

This was an intermittent problem that came on during colder weather. When it warmed up the problem went away, but it progressively got worse.....to the point that now the power loss is constant.

Other symptoms: at least a 4 MPG fuel economy loss, engine runs hotter and makes more noise (not a vibration), and it feels like I'm towing something heavy.

Things I'm considering: bad fuel injector(s), distributor (starts fine, though), fuel pump (?), transmission-related (?). Other than that I'm at a loss. Thanks again.
 
I would put a pressure gage on the fuel rail, you can have a split hose at the fuel pump.
The smell and noise and higher temperatures tells me your running lean, that would be much more noticeable in cooled weather.

Even though your running lean you can still use more fuel because your not making power and have the throttle open more.
 
langer1, I agree with your assessment. That's why I checked the fuel pressure early on and it checked good. However, I think I may recheck it just to be sure.

I also considered pulling the fuel pump for a visual inspection. It's been a while since I looked into it, but do you know how difficult it is to pull the pump from a '92?
 
I only did the leak down test and fuel regulator pressure test. Both checked good. The fuel pump has always worked (hear it when keyed before start).

That don't tell much when you have a load problem.

Also I guess you checked your vacuum at load?
 
Here's the latest update. Two days ago I started the XJ and immediately notice that it's running quiet. I also notice the RPM gauge reads just under the first hash mark above zero. Yep, that's right. It reads just a tad above zero. However, the engine is idling normally. I don't know the significance of the low RPM reading or if it's even related to the power loss.

So I drive off to work and notice I have more power. It's running so much better! Here are the other things I noticed:
  • It coasts down hills normally again - Before when coasting it acted like I had downshifted into a low gear
  • Exhaust is more quiet - before it sounded louder and labored, if that makes sense
  • The exhaust out of the tailpipe is noticeably hotter
  • Gas mileage is better
I was about to give up after doing the following:
  • Replaced all sensors (don't remember if I replaced MAP, though)
  • New tune parts several times over
  • Replaced cat
  • Replaced fuel press. regulator
  • Replaced fuel filter
  • Checked vacuum
  • Check fuel pressure and performed leak down test
  • Rebuilt head
  • Fixed exh. manifold
  • Disconnected exhaust before cat
  • Had several shops look at it
I considered swapping the tranny, but now I don't think that's it.

So I'm left with a couple of ideas. Could there be something stuck in the pipe between the collector and the cat? Could something be amiss with the valves? Could the ECM be whacky? Any other ideas?

This all started in October 2003 and very tiring to deal with. If anyone comes up with the fix I'll buy them something nice. :)
 
Have you checked ignition timing? Not sure what sensors the '92 has to determine cam location, or what kind of ignition module you have, but late timing would cause your symptoms. I beleive your tach signal comes from the ignition module--could be a clue. Also, if your engine has an EGR system, it can cause similar symptoms.
 
No EGR on the HO motor. Yeah, maybe the RPM issue is related and points to the ECM or related wiring. I inspected the connector & pins to the ECM and looked for evidence of wire chaffing in the wire bundle. I've never taken the ECM apart so don't know what to expect, but figure it's worth it to inspect for bad connections or a cracked card. It's so nice to drive now that it has decent power. :)
 
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