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Bogging and backfiring


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#1 Voodoochiled445

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Posted 10 November 2009 - 12:06 AM

I spent a good amount of time looking through searches but Id like to post a new thread with my exact symptoms.

Here is what I have done to my '87 Comanche 4.0 5 speed:
New cap and rotor
New plugs and plug wires
New oil and oil filter
New fuel filter
New alternator
Seafoam in intake and gas and tranny fluid through intake(before putting in new O2)
New air filter
New O2 sensor

OK so the truck is running a lot better then when I first purchased it but I am having some intermittent bogging issues and backfiring. Sometimes the truck runs real smooth and crisp and then sometimes it feels like something is just not all there. Doesn't run bad but just doesn't feel "crisp". Today I was on the highway doing about 2,500-3,000rpms in 4th and then whenever I let off or gave it gas it began to bog and miss real bad and backfire repeatedly. After that episode it backfired a few more times only when letting in the clutch to shift, it then drove fine for the rest of the day. I checked the coil and got a decent spark, wasn't blue but I have cheap wires. That wasn't what concerned me but what did is I noticed it needed to crank a few times before the coil produced a spark, every coil iv seen got a spark instantly the ignition was tapped but those were old school points and condenser style ignition set ups. :wall: :wall: :wall:

Last I noticed my throttle response was WAY better if I slowly press the gas, if I just mash it, it takes forever to climb RPMs.

Any help and tips would be very appreciated :wall: :wall: :wall:

btw I have cleaned many grounds including battery negative to engine (that was scary dirty)
Stock 1987 Jeep Comanche 4.0L

#2 Oizarod115

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Posted 10 November 2009 - 12:12 AM

our trucks don't remember where the crank was then they were shut off, so before it will fire the engine has to read the crankposition sensor first, depending on how far the flywheel tooth was from that sensor you may have a delay in the spark there which would be normal i would guess. (part of why they take so long to start)

another thing to check would be with a multimeter, hook up to the throttle position sensor backprobing the signal wire, and check for smooth operation and also for .8ish volts at closed throttle.

your TPS should read from .8-.9 at closed throttle, and change SMOOTHLY from that to about 4.8 or so at WOT.
if you can't adjust it to get .8 at idle, or it jumps erratically when changing throttle positions, you need one.
89 MJ 4x4 AX15 d44/30 Back on DD duty!
LIFTED! 44rear installed, original leafs, TJ d/s yoke, 6.5" coils, 33" BFG KM2s
intake swap completed, now running 2001 intake, 95 header, 50series flowmaster!
AX15 swap completed! have a 90 XJ ax15, 99 sealed yoke t-case
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#3 jmderyke

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Posted 10 November 2009 - 01:50 AM

I am new to jeeps but i have been working on cars for some time now. It sounds like you gave the mj a good tune-up ( that will certainly help) but maybe left some things out. Bogging and backfiring is a common problem with emission control problems, they can range from just a simple vacuum leak to faulty Cat. converter. But in your case i would bet it is a vacuum leak of some sort, check your EGR valve!! it is commonly overlooked and they go bad very often and people don't realize it. Look for vacuum leaks around the intake and exhaust, this can cause a bad fuel mixture and backfiring also. Pull out your service manual and got to the trouble shooting guide it can direct you to the right path.
My Comanche rattle can, nicely named Lucy
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#4 Voodoochiled445

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Posted 10 November 2009 - 02:11 AM

Thanks for the info!

Ill need to get a multimeter, been needing one a lot but never got around to buying one, I will break down and get one.

Iv tried chasing down vacuum leaks by spraying starting fluid around any vacuum source and get no gain in idle speed or rpm so EGR might make sense since its not always getting vacuum if I understand it correctly.

Been nose deep in my Chilton but a lot of it is kinda lack luster.
Stock 1987 Jeep Comanche 4.0L

#5 Voodoochiled445

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Posted 10 November 2009 - 02:14 AM

Also I have no emissions where I live so I'm wondering if I can just delete the EGR for now. I know it can be done on an old school engine by plugging the vacuum lines but this car has a brain (computer), so can I plug them and not make it mad?
Stock 1987 Jeep Comanche 4.0L

#6 garyindiane

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Posted 10 November 2009 - 04:46 AM

Also I have no emissions where I live so I'm wondering if I can just delete the EGR for now. I know it can be done on an old school engine by plugging the vacuum lines but this car has a brain (computer), so can I plug them and not make it mad?


That's what I did. Mine would not even stay running. I pulled the line, pluged it, and it smoothed right out.

-Gary

#7 jmderyke

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Posted 10 November 2009 - 11:03 AM

yeah you can take it off just make a block plate and gasket that matches to block off intake and exhaust holes. If you just cap it, it can still suck air into the exhaust from the bad diaphragm on egr valve so fabricate a block off plate or buy one. thats what i did to mine since a new egr for a 86 runs about 100 bucks f that.
My Comanche rattle can, nicely named Lucy
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#8 MancheKid86

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Posted 10 November 2009 - 11:19 AM

you can always shorten your vacume hoses a bit, over time the ends of them can get worn down and split,
by shortening it a bit it will make them have a newer end to grip/seal on

 

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#9 Voodoochiled445

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Posted 10 November 2009 - 05:19 PM

Well I plugged up my vacuum lines to the EGR and plugged the EGR nipple and I still get a back fire while pushing in the clutch at any RPM higher then 2500rpm.

I have no vacuum leaks that I can find, I have sprayed starting fluid everywhere a vacuum is present, in the engine bay and lines connecting to the tranny.

I have altered the CAD to stay engaged ala the guide found on this forum, so that vacuum motor is bypassed and lines are capped.

Seems this has been happening since the new o2 sensor, brand new bosch. Knowing my luck I bought a bad one...
Stock 1987 Jeep Comanche 4.0L

#10 Oizarod115

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Posted 10 November 2009 - 06:43 PM

o2 sensor normally will not cause bogging or backfiring on our trucks.

you still need to check the TPS sensor
89 MJ 4x4 AX15 d44/30 Back on DD duty!
LIFTED! 44rear installed, original leafs, TJ d/s yoke, 6.5" coils, 33" BFG KM2s
intake swap completed, now running 2001 intake, 95 header, 50series flowmaster!
AX15 swap completed! have a 90 XJ ax15, 99 sealed yoke t-case
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#11 Wildman

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Posted 10 November 2009 - 09:02 PM

My first question.........How many miles on this '87????

Reason I ask, almost the most likely cause of back firing is the timing is off, and on the 4.0, the timing is controlled by the ECU. No timing marks to check and NO adjustment to the dizzy. The only way the timing can be off is by a stretched timing chain. High mileage = timing chain bad.

A bad EGR valve on the 4.0 will just about choke the engine right out. Plugging the vacuum line will not have any effect on the operation of the 4.0.

Been nose deep in my Chilton but a lot of it is kinda lack luster.


Loose the Chilton's, the Hanes manual is much better, you can pick one up on e-bay for peanuts, that is about as close to the FSM that you can get.

#12 Voodoochiled445

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Posted 10 November 2009 - 09:51 PM

odometer reads 161,318

I hate cars with computers!!! haha

I was thinking timing chain, timing is usually the cause of backfiring so my first guess awhile ago was maybe a bad knock sensor. It feels like it isn't advancing fast enough, almost like when a vacuum advance isn't working properly.

Damn sensors! haha :wall:
Stock 1987 Jeep Comanche 4.0L

#13 Voodoochiled445

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Posted 10 November 2009 - 11:32 PM

Ok so I have the 5 volts with the ignition on coming from the wiring harness, now how do I check the sensor? The little positive and negative leads don't fit in the tps with the harness connected...

I'm kinda confused :huh???:
Stock 1987 Jeep Comanche 4.0L

#14 Oizarod115

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Posted 10 November 2009 - 11:36 PM

Ok so I have the 5 volts with the ignition on coming from the wiring harness, now how do I check the sensor? The little positive and negative leads don't fit in the tps with the harness connected...

I'm kinda confused :huh???:

poke them in from the backside of the connector, if you have to get some paper clips and shove in there from the back with it plugged in.
89 MJ 4x4 AX15 d44/30 Back on DD duty!
LIFTED! 44rear installed, original leafs, TJ d/s yoke, 6.5" coils, 33" BFG KM2s
intake swap completed, now running 2001 intake, 95 header, 50series flowmaster!
AX15 swap completed! have a 90 XJ ax15, 99 sealed yoke t-case
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#15 jmderyke

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Posted 10 November 2009 - 11:52 PM

I'm sorry to say this but i bet 1 million dollars that its not the timing chain, because it would be running poorly all the time. I looked at the trouble shooting guide and here is what it said:
1.egr not functioning properly
2. Ignition timing
3. Thermostatic air cleaner not working properly
4.vacuum leak
5.damaged valve springs/ valve
6. Intake air leak

You said it backfires during shifts around 2500 rpm so now we know its not mechanical cause if it was it would be doing it every power stroke (backfiring all the time). so timing chain not likely unless you heard it slip, and the engine would run very poor threw most of rpms. I have a test you should do.. if you have a mityvac hook it up to the top of the egr valve( if you don't have a mityvac get a piece of vacuum hose and use your mouth) and apply suction (about 10inch lbs of vac.) now it should hold it for at least 20 sec. if it doesnt the diaphragm in the egr valve has a hole in it. ( if there is a hole in it, that would be the answer to your backfire at 2500rpm because air will get into the exhaust threw the hole in the diaphragm even if you put a cap on top). :typing: please let me know if it works if not i will get another test for you o yeah check to see if you got a plug wire mixed up and put on the wrong cylinder {SMILIES_PATH}/jamminz.gif
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#16 Voodoochiled445

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Posted 11 November 2009 - 12:10 AM

Ok got the meter working, still trying to figure out how to read all the numbers and lines but I can say it operates smoothly from close to WOT. Not sure if its the right volts tho.

will check the EGR next.
Stock 1987 Jeep Comanche 4.0L

#17 Voodoochiled445

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Posted 11 November 2009 - 04:40 AM

Well after driving it some more I'm thinking TPS, seems the slower and steady I push the pedal the more fluid the acceleration. If I'm real steady I can bring it over 3000 RPMs and shift with no bogging or backfire. If I mash it or push real fast it will bog and backfire.

TPS? to me it really feels like the advance timing is off, what are symptoms of a bad knock sensor? past 3500RPMs the engine isn't so smooth and shakes the truck quite a bit, bad motor mounts?

truck gets to point A to point B but just doesn't feel "right".

Previous owner said it sat for many years before he owned it and he drove it very little. I don't know if that can help anyone with anything but thought Id mention it.

:wall:
Stock 1987 Jeep Comanche 4.0L

#18 garyindiane

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Posted 11 November 2009 - 07:01 AM

Hey Voodoo,

This is a good sensor diagnostics web page if you don't already know.

http://www.lunghd.co... ... ostics.htm

It may help.
-Gary

#19 Wildman

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Posted 11 November 2009 - 07:05 AM

Ok.......back to basics........What plugs did you install???

The 4.0 lives for standard Champion #4430 and loves NGK #7373, It hates any type of Platinum plug :no:

What type of "backfiring" are you getting.......thru the exhaust or thru the throttle body??

Assuming you have "Fresh gas" gas in the tank???

You mention that the truck sat for years........did you, or the PO find any mouse nests any wheres?? Mice like to chew on wires.

Here's a "little" web site that might help you diagnosis all the sensors on the Renix -

http://www.lunghd.co... ... ostics.htm

Yes, you could have a bad knock sensor throwing off the timing.

I tossed out the timing chain idea, not saying that it's the root of the problem, but almost always the caused of exhaust backfiring, but with your mileage in the upper 100's, that "could" be a cause. With the Renix 4.0, advanced and retarded timing being controlled thru the ECU, with input from the sensors, after going thru all the sensors, that check out OK, that leads back to one source. You have no idea how the truck was treated thru it's life, and someone with a very heavy right foot, whom loved to rocket launch, and run the truck at top speed, could have been in the seat. With the steel sprockets and double row timing chain the 4.0 has, the expected life is well over 300K+, unless the engine was abused.

I see garyindina beat me to the Lunghd link ;)

#20 DansGreyMj

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Posted 11 November 2009 - 10:35 AM

My Mj had a similar problem, I had to index the distributor and it is completely gone now. Chrysler even put out a TSB for it in 88.

Here are some links that should help

http://www.asashop.o.../techtotech.htm
http://blog.hemmings....e-distributor/
http://ezinearticles....tor&id=1357018
http://www.angelfire...dist_index.html
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