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1 driveability and 1 idle issue....picking brains here...

F

fordtech

NAXJA Forum User
Location
Amherst NY
98 4.0 aw4 with 98kmi.
First issue is the one that bugs me most. On light accel the jeep will sometimes hesitate and sometimes stumble (even backfired through the t-body once) for a second or two. After that it runs perfectly. Now it does this after a hot soak, and within 1/8 mile of driving. It seems to be right around the same throttle position everytime.... but I checked the tps and it doesnt have a flat spot. No CEL, but I havent taken it to get any codes pulled that might be there, that will happen in the next day or two.

Second issue is a rough idle at startup. It clears up within a minute or two. I get kinda the same thing as above for a few hundred feet after take off, but its more like its holding back alittle then is fine.

Throttle body cleaned including IAC. No vac leaks. Good fuel pressures, no leaking injectors. BG fuel cleaners run through on a regular basis. Never got a CEL. reset the PCM after any cleanings. Checked TPS with analog ohm-meter and no open found.

I concidered replacing the IAC and/ or MAP, but I cant afford to throw money at it......especially when Im spending money fixing the a/c... :(

Any insight would be great.... it would be so much easier if I had access to a scantool! but I dont....

Thanks in advance!
 
I'm having a little bit of the same with my 99, and suspect the MAP sensor of being just a little off. I had a bad MAP on my 95, and it caused stumbling under low vacuum conditions. But I haven't gotten around to fully analyzing it yet, testing fuel pressure, or cleaning the throttle body. I have a scan tool and it shows no codes.

For the rough startups, have you checked injectors? Does it smoke a little too? Sometimes an injector that doesn't close properly can dribble and flood one cylinder. This will cause rough startup until the excess burns off.
 
Matthew Currie said:
I'm having a little bit of the same with my 99, and suspect the MAP sensor of being just a little off. I had a bad MAP on my 95, and it caused stumbling under low vacuum conditions. But I haven't gotten around to fully analyzing it yet, testing fuel pressure, or cleaning the throttle body. I have a scan tool and it shows no codes.

For the rough startups, have you checked injectors? Does it smoke a little too? Sometimes an injector that doesn't close properly can dribble and flood one cylinder. This will cause rough startup until the excess burns off.
Click to expand...

Yep, it hold fuel pressure over night....not a leaky injector.

let me know if you replace the map and get any positive results.
 
How did you check the TPS? I'm asking because of the proliferation of digital multitesters...

The BEST way to check variable resistance for flat spots or "smooth" action is going to be with an analog meter, not digital. Most digital meters feature "buffer" circuits so the display can keep up with the readings, and you could skip right over a flat spot on a rheostat/potentiometer without knowing it.

A digital is a good thing, and useful for most things, but there are just times when you have to take the "low-tech" approach...

5-90

EDIT - As far as the other issue (rough idle,) I appanrently haven't said this enough - "Check for vacuum leaks." Search for my posts on the subject here, I honestly can't go into it all again (and I should probably just write up a FAQ on the subject...)

5-90
 
Last edited:
5-90 said:
How did you check the TPS? I'm asking because of the proliferation of digital multitesters...

The BEST way to check variable resistance for flat spots or "smooth" action is going to be with an analog meter, not digital. Most digital meters feature "buffer" circuits so the display can keep up with the readings, and you could skip right over a flat spot on a rheostat/potentiometer without knowing it.

A digital is a good thing, and useful for most things, but there are just times when you have to take the "low-tech" approach...

5-90

EDIT - As far as the other issue (rough idle,) I appanrently haven't said this enough - "Check for vacuum leaks." Search for my posts on the subject here, I honestly can't go into it all again (and I should probably just write up a FAQ on the subject...)

5-90
Click to expand...

DID YOU READ MY POST?
fordtech said:
Throttle body cleaned including IAC. No vac leaks. Good fuel pressures, no leaking injectors. BG fuel cleaners run through on a regular basis. Never got a CEL. reset the PCM after any cleanings. Checked TPS with analog ohm-meter and no open found.
Click to expand...

Seems like you glanced at it, went to yourself 'here we go again' and went off on your little tangent.
Ive got some experience in the auto field, but without all the resources of a shop (not in the field anymore), it is much harder for me to diag these issues. The other reason I ask is if someone else had this issue they could contribute their methods of repair or actual fix.

Im very aware how and when to use a analog vs. digital multimeter. Judging by how you replied you appearently have said 'check for a vacumn leak' enough before, so no you dont have to go into it all again.

Thanks for reinforcing what I have already done, but please read the post before you reply.
 
Last edited:
Sounds similar to a problem Im having. Check out this thread for some things Ive tried and didn't work, and some other suggestions.

http://www.naxja.org/forum/showthread.php?t=88728

If you figure it out definately post up cause all to often people fix the problem but never say how, making it quite hards to search when all you can find is questions but no answers.
 
98XJSport said:
Sounds similar to a problem Im having. Check out this thread for some things Ive tried and didn't work, and some other suggestions.

http://www.naxja.org/forum/showthread.php?t=88728

If you figure it out definately post up cause all to often people fix the problem but never say how, making it quite hards to search when all you can find is questions but no answers.
Click to expand...

Mines not that bad....and it happens as soon as I start it cold. It idles at normal RPM's, just roughly. It clears up as soon as I pull away or rev it.

Thanks!
 
I had a similiar problem (stumbling/backfire/rough idle) on my 98. After replacing the tps, and o2 sensors and several months of trying to figure it out I found the problem. The ground for the o2 sensors on the engine block would get wet or dirty. That would cause it short and blow the 15A fuse that is under the hood. I've seen that fuse also blow when the exhaust was smashed on rocks or the o2 sensors riped out. With the fuse and short fixed it ran fine.
 
67TEMP said:
I had a similiar problem (stumbling/backfire/rough idle) on my 98. After replacing the tps, and o2 sensors and several months of trying to figure it out I found the problem. The ground for the o2 sensors on the engine block would get wet or dirty. That would cause it short and blow the 15A fuse that is under the hood. I've seen that fuse also blow when the exhaust was smashed on rocks or the o2 sensors riped out. With the fuse and short fixed it ran fine.
Click to expand...

did it cause a check engine light? I'll look at the grounds...and double check the fuse, which should be the heater circuit.


OH YA, small update in the whole saga... Autozone cant do much with their scan tool if the CEL isnt on... so Im at square one from that perspective.
 
Last edited:
fordtech said:
DID YOU READ MY POST?


Seems like you glanced at it, went to yourself 'here we go again' and went off on your little tangent.
Ive got some experience in the auto field, but without all the resources of a shop (not in the field anymore), it is much harder for me to diag these issues. The other reason I ask is if someone else had this issue they could contribute their methods of repair or actual fix.

Im very aware how and when to use a analog vs. digital multimeter. Judging by how you replied you appearently have said 'check for a vacumn leak' enough before, so no you dont have to go into it all again.

Thanks for reinforcing what I have already done, but please read the post before you reply.
Click to expand...


OK, I missed the bit about analogue ohmmeter - I'm so used to dealing with digitals - but did you also check for vacuum leaks everywhere? The throttle body doesn't usually develop leaks because it's relatively cool out there - but manifold mounting surface leaks are like fleas on dogs - damn things are everywhere. Same test to see if they're happening - or you can just do what I do, and retorque them as part of your annual inspection anyhow. I honestly find, every time, one or two that have worked a bit loose and need to be nipped up - and it happened early enough and often enough that I don't consider it time wasted to check them every year.

And, to be perfectly blunt, most people honestly don't think to check the manifold mating surfaces for vacuum leaks - on anything. They're very common on inline sixes (has to do with the different materials - aluminum expands twice as fast as iron/steel, and with the sealing surface being so long...) so my advice on that still stands.

I do often read this board while I'm doing other things - usually unrelated - so it's possible for me to make an honest mistake every now and again. I think I was doing research for a term paper on the Maya/Aztec people while I responded the first time, which explains why I'd missed a few things. Still, it was useful advice, and I was trying to confirm what you'd done (just because I'd missed it the first time was no reason to jump - cool off a bit, willya? I've got enough heat to deal with here as it is...) and I'd need to do some reading to see what else might be causing it. However, those are good first checks, and should be done in any case. Also, check all your engine management vacuum lines - they're Nylon, and crack with age. How much age? Can't really say - it's sort of random. I've seen them go 20 years, and I've seen them go 20 months. Hard tellin'.

5-90
 
5-90 said:
OK, I missed the bit about analogue ohmmeter - I'm so used to dealing with digitals - but did you also check for vacuum leaks everywhere? The throttle body doesn't usually develop leaks because it's relatively cool out there - but manifold mounting surface leaks are like fleas on dogs - damn things are everywhere. Same test to see if they're happening - or you can just do what I do, and retorque them as part of your annual inspection anyhow. I honestly find, every time, one or two that have worked a bit loose and need to be nipped up - and it happened early enough and often enough that I don't consider it time wasted to check them every year.

And, to be perfectly blunt, most people honestly don't think to check the manifold mating surfaces for vacuum leaks - on anything. They're very common on inline sixes (has to do with the different materials - aluminum expands twice as fast as iron/steel, and with the sealing surface being so long...) so my advice on that still stands.

I do often read this board while I'm doing other things - usually unrelated - so it's possible for me to make an honest mistake every now and again. I think I was doing research for a term paper on the Maya/Aztec people while I responded the first time, which explains why I'd missed a few things. Still, it was useful advice, and I was trying to confirm what you'd done (just because I'd missed it the first time was no reason to jump - cool off a bit, willya? I've got enough heat to deal with here as it is...) and I'd need to do some reading to see what else might be causing it. However, those are good first checks, and should be done in any case. Also, check all your engine management vacuum lines - they're Nylon, and crack with age. How much age? Can't really say - it's sort of random. I've seen them go 20 years, and I've seen them go 20 months. Hard tellin'.

5-90
Click to expand...

Thanks. I have checked all lines, and the intake mateing surface with t-body cleaner, and also a butane torch (not lit of course!).

I appreciate your response and appoligize if you thought I was jumping on ya... I wasnt, just straightening out the facts.
 
I did get a CEL but occasionally it would turn off for days at a time but the problem would still be there.

The fuse for the sensor heater is a 15A in the row next to the relays, in the fuse block under the hood.
 
This sounds suspiciously like an intermittant MAP sensor problem to me. I think it's located directly under the throttle body on a 98. I'd check the wiring terminals going to it, and maybe the mounting gasket. Might also be the sensor itself going bad; a MAP related code would confirm it.....
 
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