Power Loss

I've been chasing down an engine power loss problem and the fix has eluded me for some time. I'll tell you the symptoms first and then what's been replaced or checked.

Symptoms:

• Obvious seat of the pants power loss at all speeds
• Engine runs noisier (exhaust louder)
• RPM steady throughout (no missfires)
• At least a 3-4 MPG loss
• Engine runs hotter than normal
• It feels like I'm towing something heavy
• Lose speed coasting down hills that normally cause it to gain speed
• Just recently the RPM at idle is reading near zero on the gauge, but there's no perceived loss in RPM
• No stumble or hesitation
• Auto tranny shifts later than normal and it's a hard shift

The power loss is intermittent. Very occasionally the power comes back and it drives great. It gives no fault codes.

What's been done:

• Every sensor replaced (except for CPS in distributor)
• All new tune up parts (many times over)
• Rebuilt head
• Good seal on intake/exhaust manifold
• Replaced cat
• Replaced computer
• Checked continuity between sensors and PCM connector
• Good coil
• Fuel system pressure checked
• Replaced fuel pressure regulator
• Replaced fuel filter
• Passed emissions (figure compression good as a result)
• Disconnected exh. pipe before cat and ran it w/ no change
• Seafoam treatment
• Ran can of BGK44 in fuel

Things that are left that might be the cause:
Fuel pump - symptoms are usually different though
Distributor - not sure about this
Injectors - wouldn't a faulty injector cause a bad emissions reading?
Exhaust - something stuck in pipe between manifold and cat?

Anything else worth checking? Thanks!

Does it pull to one side or another?? How about different noises when stopping/turning/accelerating?? I'm thinking bearing/brake drag....

does it loose speed when in N downhill?

im wondering if its something in the drive line. but it should be making a noise if so, i would think.

It's definitely engine-related, to include the wiring. I know if the problem is present or not immediately after starting the engine.

Right now I'm thinking it's likely chaffed wiring somewhere in the harness due to it being intermittent and the faulty RPM reading at idle, but wanted to exhaust all possibilities.

You said "every sensor replaced", that would include TPS. Is yours adjustable?? Sounds like some of the symptoms I've got with mine....especially with the shifting issues.


Also, try cleaning and re-grounding. You could have an intermittent ground issue. Would be easier to track down than chafed wiring....=P

RaccoonJoe said:

You said "every sensor replaced", that would include TPS. Is yours adjustable?? Sounds like some of the symptoms I've got with mine....especially with the shifting issues.


Also, try cleaning and re-grounding. You could have an intermittent ground issue. Would be easier to track down than chafed wiring....=P

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Yes, I replaced the TPS and it's the non-adjustable type. I checked the voltage a while back and cleaned it for good measure (not that it was dirty). That's a good idea to check for an intermittent ground. Thanks!

I would look into the source of the RPM signal for your year model, source of the data, the computer or a distributor sensor? and who uses that data, i.e. the transmission comuter!

Also check for lose connections/connectors. Did you replace the muffler too?

The FSM isn't too helpful about how RPM is measured. It says the computer generates the signal. I know there's more to it than that, but I don't have the info. The low RPM reading happened for the first time about 4 weeks ago when the power magically came back. So I figured it's related somehow. However, the power loss is back, but the RPM still reads low at idle.

Also figuring that the engine runs hotter than normal and that it gets worse MPG, it seems to me it's running lean. Maybe a bad fuel injector or the associated wiring? However, wouldn't the computer store a code if an injector were faulty?

I replaced the coolant temp sensor and just recently tested it in accordance with the FSM, to include continuity checks between the sensor and the computer's 60 pin connector....all checked good.

Several shops have taken a crack at this to no avail, so it's definitely a stumper.

A far off shot but did you check the bearings in the rear end. It sounds like the same problem I had just before I lost one out of a "90 sport. Go for a drive and put your hand on the dif cover. If its warmer then usual..bingo. Hope that helps.

The code system like everything else is not a perfect science. The error codes are just a source of early diagnostic data, not a cure all.

The RPM problem could be the gauge, the wiring from there to the computer, the computer or the CPS or synch sensor (I think there is still a synch sensor in there somewhere on that year, but not sure, was in the distributor in earlier years.) The RPM gauge problem may be unrelated, or it may be a sign of a computer, or signal to the computer problem, and thus may be related to the power problem.

Also there is no CPS in the distributor, it is on the transmission bell housing near the flywheel. The sensor in the distributor is I think called a synch sensor. It could be your probelm!!!! I seem to recall that these engines still run when it goes bad, but don't perform well.

I still ask, is the muffler also new???? If not it could be the power loss problem!

Rereading everything here has me thinking Muffler is cloged up and causing most of your symptoms!

Ecomike said:

The code system like everything else is not a perfect science. The error codes are just a source of early diagnostic data, not a cure all.

The RPM problem could be the gauge, the wiring from there to the computer, the computer or the CPS or synch sensor (I think there is still a synch sensor in there somewhere on that year, but not sure, was in the distributor in earlier years.) The RPM gauge problem may be unrelated, or it may be a sign of a computer, or signal to the computer problem, and thus may be related to the power problem.

Also there is no CPS in the distributor, it is on the transmission bell housing near the flywheel. The sensor in the distributor is I think called a synch sensor. It could be your probelm!!!! I seem to recall that these engines still run when it goes bad, but don't perform well.

I still ask, is the muffler also new???? If not it could be the power loss problem!

Rereading everything here has me thinking Muffler is cloged up and causing most of your symptoms!

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I'm fully aware that the diagnostic system isn't perfect. It's just one more bit of information.

Yes, there are two CPS units. One is the crankshaft position sensor (bellhousing) and the other is the camshaft position sensor pickup (dizzy). When the CPS in the distributor goes bad it usually means long crank times, which is not the case and the reason why I never changed it.

The Flowmaster muffler was installed maybe 5 or 6 six years ago. These mufflers use baffles, which means they're essentially hollow inside, so the likelihood of something getting clogged in it is remote. I have thought about it, though. That's why I had a shop disconnect the exhaust before the cat and then drove around to see if it improved. It didn't. Like you, I keep coming back to the exhaust. It certainly acts like something is clogging it, based on the power loss and the exhaust note change. I just can't imagine anything lodging itself in the pipe between the manifold and the cat.

As for the RPM issue, it happened at the exact time my power came back. Coincidence? Maybe. The RPM readings are overwise spot on; it's only at idle that it reads near zero. I'd like to know what device provides the computer with RPM data.

Chaffed wiring could explain why the power loss has been intermittent, seeming to come and go at random. It's certainly indicative of an electrical connection problem. And if there are more chaffed wires all in the same area, that could explain the RPM issue as well.

BTW, as I mentioned earlier, I swapped out the computer and troubleshot the wires from the sensors to the computer's 60-pin connector.

Baffle plates in mufflers can suddenly break loose and sandwhich one on top of another. When this happens it causes a noticable restriction and power loss. Could you have had two problems, solved one and had the muffler fail after the exhaust system test but while you were fixing the first problem? Sometimes you need to go back to square one. Just a thought. But sounds like you are the right track already.

Only other thought I have is fuel injectors. When I bought my 87 Renix I did a lot of work on it. It had power problems, overheating problems, idle speed problems, but long story short one of the most significant improvements (I just remembered this) I made was replacing the used assortment of original and junk yard pulled fuel injectors I had. They were leaking too (another story), but when I replaced them all with a "matched set" of new aftermarket four hole injectors from www.fiveomotorsports.com it went from extreemly anemic (barely ran power wise, top speed was like 60 mph, if that) to say 65% of what a new engine should put out. A new muffler fixed another 20% of the power loss problem in my case.

I got to thinking that if the flow rates were not closely matched (some injectors were worn more and were flowing better than others) then some cylinders were running rich and some lean since the computer can not see the individual injector A/F ratio results of each opening pulse signal, it more or less averages them out as it sees an average A/F mix in the exhaust pipe (I think? I don't think they can do the timing calculations needed to distinguish when a fired cylinders exhaust gases are passing the O2 sensor, it is too complex, too many variables.....). So you might end up with 3 running rich, 3 running lean and lossy power output as a result. If it had 6 O2 sensors and 6 exhaust pipes the computer could manage each fuel injector individually, but in realty these things are looking at a moving A/F ratio average, adjusting the next fuel injector based on the last reading from the O2 sensor.

Therefore, a few off injectors could severly affect power performance.

May not be your problem, but was a big part of the poor power problem with mine early on. It went from about 50 hp at WOT to about 120 HP at WOT with just new, certified and tested, "MATCH" (flow rate) fuel injectors from Five-O.

Replacement injectors from FiveMotorSports is already planned.

If an injector is firing incorrectly, has a bad spray or is otherwise operating improperly, wouldn't it be evident in the engine idle or during an emissions test?

The power loss is intermittent. Very occasionally the power comes back and it drives great. It gives no fault codes.

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My guess is tranny related. A leaky seal in the pump can act like what you describe.

old_man said:

My guess is tranny related. A leaky seal in the pump can act like what you describe.

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Early on I thought the tranny might be involved, but I can tell whether the power is there or not just from starting the engine and hearing how it sounds. So it's definitely engine related.

Tom R. said:

Replacement injectors from FiveMotorSports is already planned.

If an injector is firing incorrectly, has a bad spray or is otherwise operating improperly, wouldn't it be evident in the engine idle or during an emissions test?

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Yes , but if some are a lot older, more mileage than others, the older ones will have more wear and deliver more fuel for a given pulse width making one cylinder run richer than the next. The computer, O2 sensor and Cat may be able to compensate enough to pass emissions, don't know for sure. In my case, mine was so bad it would have failed emissions tests anyway as it had a lot of other warn out parts all over it at the time.

Sounds like you have something weird and hard to describe going on. Can you tell us more about this difference in start up being an indication of whether it will have power or not. Do I understand you correctly that this power loss problem is INTERMITENT?

I once had an exhaust pipe delaminate internally and the delaminated part was still attached to the inside of the exhaust pipe wall, so it acted like a variable damper door, periodically partially blocking enough of the exhaust path to kill the power! That was a weird one too.

Could there be something of a variable nature periodically plugging the throttle body, air filter or intake air stream somewhere? Maybe someone left a rag in their, LOL?

Ecomike said:

Yes , but if some are a lot older, more mileage than others, the older ones will have more wear and deliver more fuel for a given pulse width making one cylinder run richer than the next. The computer, O2 sensor and Cat may be able to compensate enough to pass emissions, don't know for sure. In my case, mine was so bad it would have failed emissions tests anyway as it had a lot of other warn out parts all over it at the time.

Sounds like you have something weird and hard to describe going on. Can you tell us more about this difference in start up being an indication of whether it will have power or not. Do I understand you correctly that this power loss problem is INTERMITENT?

I once had an exhaust pipe delaminate internally and the delaminated part was still attached to the inside of the exhaust pipe wall, so it acted like a variable damper door, periodically partially blocking enough of the exhaust path to kill the power! That was a weird one too.

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I can tell whether I have power at start up by two things. First, the exhaust is noticeably quieter, both inside the cab and outside the vehicle. And it's a different exhaust note, meaning that besides volume level it also sounds different. Second, I can feel through the driver's seat that the engine is running different. Maybe there are less vibrations. The engine also revs easier.

The best way to describe the problem at its worse: it feels like I'm towing something VERY heavy. There have been times I had to redline it just to get it moving half-way decent. Mileage went from a consistent 18 to about 11 or 12 MPG.

Yes, the problem has been intermittent.

A quick history since problem first surfaced. About 3.5 years ago I was driving on the highway and noticed a marked decreased in engine power. The XJ had a hard time pulling highway inclines and engine temp was running almost up to the 3/4 mark on the gauge. It had run great up to this point.

The problem lasted the rest of the fall and through the winter. When warmer temps came the next spring I noticed the power came back. I figured the temps had something to do with it, so I paid close attention. The problem surfaced periodically during the summer and became more frequent the following fall as the temps decreased. That winter I had the power loss the entire time. From that point on it was rare to get the power back. My XJ sat unused for a year until 7 months ago when it became my daily commuter again.

About four weeks ago the power suddenly came back in all it's glory. Engine noise went way down, had much better get up and go, mileage improved, and tranny shifted smoother. This is when the RPM issue surfaced. Thanks.

No doub't about it, that is an intermitently blocked exhaust. It has to be. Take it to another exhaust shop, or 2 and have them go over it from the exhaust manifold itself to the end of the tail pipe.