# HELP!  Jeep problem driving me bonkers!



## quads (Aug 31, 2010)

The vehicle:  1991 Jeep Comanche, 2.5 Liter multi-port fuel injection, 4 speed manual transmission

The symptoms:  mid-range problem, engine jumps, bucks, cuts out, won't accelerate, occasional popping.  If I baby it and just go real easy on the throttle, even lugging the engine a little as I shift gears, I can get it to gradually run all the way up to highway speed without it missing a beat.  But, if I push the throttle into mid-range, it does all of the above, in any gear.  Or, if I stomp the gas peddle tight to the floor and hold it there, the engine levels out and will accelerate, until I ease off to mid range again, and it immediately starts acting up.  Barely runs at mid range throttle.  Once at highway speed, it will cruise smoothly with no problem, until I push the pedal to mid range.

New parts installed so far:
Treated with fuel injector cleaner
Fuel Filter
Crankshaft Position Sensor
Throttle Position Sensor
Spark Plugs
Spark Plug Wires
Distributor Cap
Distributor Rotor
Ignition Coil

What's left?  Thanks in advance!


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## ANeat (Sep 1, 2010)

Pretty tough to tell if its a ign or fuel problem.  Do you get any check engine or fault lights?

What else ??  O2 sensor is another thing,   You should be able to test it somehow without just replacing stuff.


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## oilstinks (Sep 1, 2010)

just for giggles go ahead and clean the mass air wire if it has mass air. Mass air cleaner bout 7 bucks a can and can really wake up some power if never cleaned some claim 5hp. im suspecting maybe fuel pressure or a spark timing issue. How many miles on motor?


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## PapaDave (Sep 1, 2010)

Hey quads, try jeepforum.com. Those guys know Jeeps inside and out, and will probably be able to figure it out for you. 
Since it has a distributor, have you checked the timing?


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## begreen (Sep 1, 2010)

Are you getting a check engine light? If so, have the codes checked. Off the cuff, these are areas to check: 
First suspect: Throttle Position Sensor - http://www.wonderhowto.com/how-to-test-and-replace-your-cars-throttle-position-sensor-267168/

If OK, do a pressure test on the fuel system to check the Fuel Pump and Fuel Pressure Regulator


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## quads (Sep 1, 2010)

Check engine light has been on since about 1996, 100,000 miles ago.  It has 140,000 on it now.  I tried the key on off on off thing but the light didn't give me any useful codes.  I tested the crank sensor with an ohm meter and it was bad as compared to the readings on the new one.  It had a hard starting problem also, which the new crank sensor cured, but it did nothing for the mid range problem.  I intended to test the throttle position sensor, but I couldn't get to the terminals without removing it first and one of the torx head screws 'rounded-out'.  In the process of removing the rounded screw, I inadvertently destroyed the old sensor and had no choice but to replace it anyway.  It had an up and down idle too, which the throttle position sensor cured, but also did nothing for the mid range problem.

It doesn't have a mass air flow sensor.  I'm told that 1990 was the last model year for it (throttle body fuel injection) and my 1991 with multi-port fuel injection eliminated it.  If the ignition timing has changed, it only changed because of a failed part, the distributor hasn't moved.  It idles fine, runs at low throttle good, and at wide open throttle.  Between low throttle and wide open is where the trouble is.

I have been visiting the Jeepforum, which is where I got the ideas to change the parts that I already did.  Lots of good info there, but still stumped.

I have been thinking about the fuel regulator, O2 sensor, and manifold absolute pressure sensor (MAP).  The O2 is original, so is most likely bad.  And I think that's what turned the engine light on all those years ago.  But it doesn't run rich.  I'm beginning to think lean is the problem (if not ignition, which I have almost eliminated).  I'm not sure if O2 sensor would cause a lean problem.  Fuel regulator is purely mechanically, controlled by vacuum, so I would think if bad it would be bad at all throttle settings.  But, I really don't know exactly the whats and whys of the regulator so I have to research it a little more.  The local NAPA store sells a fuel pressure gauge to hook to the injector rail and check the pressure, but $52!

I have almost eliminated all the ignition parts, so I guess now what I am looking for is something that would cause a lean mixture.  But not lean at idle/low throttle, and not lean at wide open.  This thing that has failed must control the fuel mixture at mid range, but is bypassed at low settings and wide open.  MAP sensor?  It has both a vacuum line and electrical connection.  I need to research that more to see what it does, but I vaguely remember reading something about it controlling fuel mixture.  So?...

Thanks everybody!  Keep the ideas coming, I hate being stumped and appreciate all the help!  I am bound and determined to figure this thing out.  When I do, I will be sure to post what the problem was.  Could be a year from now though at this rate.  Haha!


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## begreen (Sep 1, 2010)

Seems like it might be less expensive to take it to the shop and have them read the computer codes and measure the fuel pressure. That has to be better than throwing parts at the situation.


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## homebrewz (Sep 1, 2010)

It sounds like a part with a potentiometer or variable resistor is worn out right about the middle of the strip. The throttle position sensor and mass air flow sensor are obvious targets for replacement in that case. However, you have replaced the former and it doesn't have the latter. Check the MAP, but you will need to get those codes read one way or another. The truck should be OBD I and therefore easy to read.


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## gzecc (Sep 1, 2010)

Two things, I think pep boys will give codes for no charge. Have you tried Seafoam in the gas? If not, wait til your very low on gas and put some in the tank. It cleans deposits very nicely.
I get an intermittant hessitatation myself in my 2001 xterra. I have 130k with all original plugs, wires etc.
I did change the timing belt at 90k. My hesitation starts 2 minutes after startup then goes away after 5 mins.


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## ironpony (Sep 1, 2010)

Quads,
I am leaning towards a timing issue, bad gear causing excessive play
also the vacuum advance sticking, leaking, or if there is not a vacuum operated one
however it is advanced/ retarded
popping is what im going by
second choice valve timing gearset or chain worn
third actually a valve itself


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## ironpony (Sep 1, 2010)

there is also a cam position sensor which you can check


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## ROBERT F (Sep 1, 2010)

Incorrect fuel pressure, can cause stumbles, hesitation, and "bucking of the engine" at light load high vacuum, such as cruising at highway speeds with throttle partial cracked.  low fuel pressure would usually have lower than expected idle, poor acceleration, mid range driveability issues, and pinging at high rpm low vaccuum.  high pressure is rumpy idle, foul smell, surgy acceleration, cleans up as rpm increases, poor fuel economy.


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## ROBERT F (Sep 1, 2010)

Vaccuum leaks can cause some strange problems also!?!?!?!?


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## mayhem (Sep 1, 2010)

> It doesn’t have a mass air flow sensor.  I’m told that 1990 was the last model year for it (throttle body fuel injection) and my 1991 with multi-port fuel injection eliminated it.



Not sure I buy this...MAFs have become much more prvalent over the years, not less.  Look at your air intake tube, somehwere between the air filter and the throttle body there should be a coupling that has an electrical connector...thats your MAF.  Get a can of MAF cleaner, take off the MAF and hose the hell out of it.  Do not under any circumstances touch the wires, only touch the outside of the housing...touch the wires and you're gonna buy a new MAF.  Again, this assumes you have a MAF in the first place.

Additionally I recommend you do a Seafoam clenaing.  Great idea to put a bottle in the gas tank, I would do it when you fill it so its properly mixed with the gas.  Look for a good vacuum source on your motor and do the seafoam engine treatment too...instructions are on the can or their website, but basically you slowly pour about half a can into your running motor, then you shut it down or it stalls and you let it sit for about a half hour, start it and drive it like you stole it.  The goal is to clean off all the built up carbon in your motor...according to Google searches I see the old Jeep 2.5 is prone to carbon buildup and the EGR can be suspect after awhile...you might consider changing that too.

Vacuum leaks can cause all kinds of issues, as noted above.  Start it up and listen for vacuum leaks.

Could be a fuel pump too.


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## vvvv (Sep 1, 2010)

vacumn leak test i saw on tv= engine idling & running rough. use propane torch,unlit, & shoot propane around the hoses or wherever leak might be. if engine idles better, the leak is in the area where it sucked in the propane.


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## quads (Sep 1, 2010)

mayhem said:
			
		

> > It doesn’t have a mass air flow sensor.  I’m told that 1990 was the last model year for it (throttle body fuel injection) and my 1991 with multi-port fuel injection eliminated it.
> 
> 
> 
> ...


That's what I thought too, but there isn't anything in the intake all the way from the air filter into the (carb?) where the throttle butterfly is, no MAF.  Funny thing too, it doesn't have an EGR valve!  Makes me wonder how the crazy thing runs to begin with!


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## quads (Sep 1, 2010)

PINEBURNER said:
			
		

> Incorrect fuel pressure, can cause stumbles, hesitation, and "bucking of the engine" at light load high vacuum, such as cruising at highway speeds with throttle partial cracked.  low fuel pressure would usually have lower than expected idle, poor acceleration, mid range driveability issues, and pinging at high rpm low vaccuum.  high pressure is rumpy idle, foul smell, surgy acceleration, cleans up as rpm increases, poor fuel economy.


I just talked to a Jeep mechanic and he told me that is what it sounds like, incorrect fuel pressure.  He said as long as mice haven't plugged the muffler or cat converter, it's got to be the fuel pressure regulator or fuel pump itself.  Since it levels out at wide throttle, probably not the pump.  Exhaust system seems to be flowing freely, but is so rusting and leaking anyway that I hate to touch it.  He was leaning towards fuel regulator.  

I unplugged the MAP sensor (which adjusts both ignition timing and fuel mixture) and it made a big difference in idle and everything.  Even died at a stop sign, which it never does otherwise.  Ran even worse than it was.  Plugging it back in also made a difference, sometimes killing the engine while it readjusted itself.  I  am assuming MAP sensor is probably ok then, at least it seems to be doing something.

Unplugged the fuel regulator (it is only mechanical with a vacuum hose, no electrical connection) and it made no difference, even seemed to run a little better in some ways.  So, the fuel regulator seems to be doing nothing, or even making things worse, so I may try replacing that.

Jeep mechanic had me try the codes again (key on and off 3 times in 5 seconds and count the number of engine light flashes).  This time I got 12 which means battery has been unhooked at some time, 33 which means no A/C clutch (it wasn't made with A/C), and 55 which means end of codes.  So, nothing of value.  But of course the pressure regulator probably wouldn't give a code anyway, being mechanical only.

I would take it to a garage, but two weeks before anybody can get to it, and then they charge $200 just to look at it.  Not to mention they get full list price for parts.  So, I'll keep throwing things into it until I get it figured out and still come out cheaper.  Plus, the learning experience alone is worth something to me.  I can fix anything those guys can anyway, just a little more old-fashioned than they are.  Ha ha!


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## quads (Sep 1, 2010)

BLIMP said:
			
		

> vacumn leak test i saw on tv= engine idling & running rough. use propane torch,unlit, & shoot propane around the hoses or wherever leak might be. if engine idles better, the leak is in the area where it sucked in the propane.


It idles great, no problems at low speed/idle.


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## mayhem (Sep 1, 2010)

Maybe its time for a SBC transplant?


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## Freeheat (Sep 1, 2010)

That year wont have a mass air flow sensor but does have a MAP sensor. When you go to WOT( wide open throttle ) does it  cut out  instant or sputter. What I'm looking for is it electrical or fuel.


JIM


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## ANeat (Sep 1, 2010)

As for the check engine light most of the big chain parts stores will scan the codes for free.


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## wenger7446 (Sep 1, 2010)

I had the same problem with an early 90s Wrangler and it was a bonding strap that i removed  somewhere off the engine (sorry i can't where).


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## quads (Sep 1, 2010)

estang said:
			
		

> That year wont have a mass air flow sensor but does have a MAP sensor. When you go to WOT( wide open throttle ) does it  cut out  instant or sputter. What I'm looking for is it electrical or fuel.
> 
> 
> JIM


When I go wide open throttle it will sometimes pop and cut out as I pass mid range, but once it's pegged to the floor it levels out, albeit with not much power though.  If I back off from wide open lightly, it surges like it wants more power, but then sputters/buck/jumps.  Beginning to think it is losing fuel, but funny that wide open levels out.  Anyway, still got it tore apart and will tinker with it more tomorrow.


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## quads (Sep 1, 2010)

Oh, and I'm planning to get me a little Chevy truck for a spare, as soon as I can find one in my price range.  Ha ha!


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## Freeheat (Sep 1, 2010)

Hey Quads have you ever checked timing?? chain could have jumped a tooth.


JIM


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## quads (Sep 2, 2010)

estang said:
			
		

> Hey Quads have you ever checked timing?? chain could have jumped a tooth.
> 
> 
> JIM


I wondered that too, but according to the Chilton manual it has timing gears like the old straight 6 engines.

With all of you guys' help, I think I have it narrowed down.  I took the fuel return line off the pressure regulator and it never returns a drop of fuel to the tank, even at idle.  With my luck it's the pump that is bad/weak.  I was trying to avoid tearing the rusty old gas tank apart, but I guess I should have just assumed it would be whatever is the hardest to fix and started there!  HA!


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## mayhem (Sep 2, 2010)

Not sure where the tank is located, but on alot of trucks its easier to just pull the bed off to expose the tank and you can get right at the pump.

I had a 77 Oldsmobile 98 Regency as a winter beater when I was a lad...it had similar issues, though the sputtering got worse with WOT then half...turns out the gas line was rotted in one spot and I was sucking air periodically.  Something else to look for anyway.

I bet its the pump though.


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## tjnamtiw (Sep 2, 2010)

Did anyone mention fuel filter???  The fact that it stumbles at part throttle sounds like the throttle position sensor.  Hook a resistance meter to it after unplugging it and slowly open the throttle with engine shut off.  Hopefully you have a meter with a needle to see if it changes steadily.

A collapsed cat converter could do it too.  Put a vacuum meter on intake manifold.  Vacuum should go down as revs go up.  If it doesn't, cat is plugged.


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## begreen (Sep 2, 2010)

quads said:
			
		

> estang said:
> 
> 
> 
> ...



Please be super careful if attempting to do this. An empty fuel-tank has the potential to be a serious bomb.


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## quads (Sep 2, 2010)

FIXED!  Thanks everybody for all your help!  I am in your debt.  And the winning failed part was........or wait, maybe I should just let everybody guess?  No, it was a weak FUEL PUMP!  Now why the engine would level out at wide open throttle, is beyond me, just one of those mysteries I guess, but now that it's fixed it certainly has power at wide open throttle, which it never had before.

It needed the new crank sensor, because that immediately cured the hard starting problem.  It needed the new throttle position sensor because that cured the up and down idle problem.  I did replace the fuel filter, plugs, wires, cap, and rotor, which it probably all needed anyway.  The only thing I bought that it didn't really need was the ignition coil.  And considering I got a quote from a garage to replace the fuel pump for $600, I think I did pretty good.  It took me the better part of a week to fix it, but the garage had a waiting list of two weeks.

I bought that truck brand new off the showroom floor 20 years ago.  It really looks like crap now, all rusted out and I have hit two deer with it over the years, but it has never ran so good even when it was new as it does right now!  Unless it's just been running that bad for so long that my old brain has forgotten.  I can honestly say one thing though, I'm getting too old to be leaning over hoods and rolling around under trucks all day anymore...

Now tonight, I become a Chevy owner again.  A neighbor from a few miles away (almost all of my neighbors are a few miles away) heard about my plight and I was complaining that I needed a spare truck, and he has offered me a pretty good deal on a little Chevy truck, a few years newer than the old Jeep truck.  I haven't been a Chevy owner for at least 25 years, and this will be the first used car I've bought in that long too.  I'm not even sure how to register a car in Wisconsin anymore.  Maybe my neighbor will know.

Anyway, thanks again for all the help!  I'm tickled pink to have the old truck running up to snuff again, for now...


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## quads (Sep 2, 2010)

Oh, and I forgot to mention that the fuel pressure regulator was bad, stuck shut, and I think that's what killed the fuel pump by building up too much pressure all the time.


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## vvvv (Sep 2, 2010)

quads said:
			
		

> FIXED!  Thanks everybody for all your help!  I am in your debt.  And the winning failed part was........or wait, maybe I should just let everybody guess?  No, it was a weak FUEL PUMP!  Now why the engine would level out at wide open throttle, is beyond me, just one of those mysteries I guess, but now that it's fixed it certainly has power at wide open throttle, which it never had before.
> 
> It needed the new crank sensor, because that immediately cured the hard starting problem.  It needed the new throttle position sensor because that cured the up and down idle problem.  I did replace the fuel filter, plugs, wires, cap, and rotor, which it probably all needed anyway.  The only thing I bought that it didn't really need was the ignition coil.  And considering I got a quote from a garage to replace the fuel pump for $600, I think I did pretty good.  It took me the better part of a week to fix it, but the garage had a waiting list of two weeks.
> 
> ...


plugged gas filter makes the fuel pump work harder?


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## quads (Sep 3, 2010)

BLIMP said:
			
		

> plugged gas filter makes the fuel pump work harder?


The gas filter was the very first thing I replaced.  As far as I know, the old one was original!  But, I could blow through it just as easily as the new one.  With the pressure regulator stuck shut, that pump had to be working it's guts out because no fuel was returning back to the tank.  The pump had to be at full pressure all the time.

Just picked up a used S10 Chevy a little while ago.  Old Mrs. Quads is already in love with it!


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