98 ZJ 4L I6 Random No Acceleration - JeepForum.com
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post #1 of 22 Old 01-25-2020, 05:25 PM Thread Starter
wildbill23c
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98 ZJ 4L I6 Random No Acceleration

I've had this issue since I got the Jeep almost 3 years ago. Never does it when its cold...after it has ran a while, then shut it off get back in it and drive away you go to accelerate and it just won't accelerate no matter how hard you press on the accelerator, kind of does a horrible shudder, and a knock from under the vehicle sounds like back around the fuel tank maybe? If I just let off the accelerator and then apply the throttle again its fine, and never does it while driving afterwards. I don't get any codes and no check engine light. I thought maybe a fuel pump issue, but I'd think that it would do that all the time if that were the case.

98 Grand Cherokee SE 4L I6 4x4.

I haven't had anything done to it yet as far as a tune-up, etc. as I feel that isn't the problem. Is this a heat soak issue since it happens after it has set for a short period of time after a somewhat long drive?

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post #2 of 22 Old 01-25-2020, 05:36 PM
jtec
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you are using a scanner to check for codes?

If no codes with scanner -
Might want to check fuel pressure - use a gauge.
Try an exhaust back pressure test. Easy a backpressur gauge temp in o2 sensor bung.
Try CAREFULLY cracking open the gas cap - any rushing air sound?

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #3 of 22 Old 01-25-2020, 05:48 PM Thread Starter
wildbill23c
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Quote:
Originally Posted by jtec View Post
you are using a scanner to check for codes?

If no codes with scanner -
Might want to check fuel pressure - use a gauge.
Try an exhaust back pressure test. Easy a backpressur gauge temp in o2 sensor bung.
Try CAREFULLY cracking open the gas cap - any rushing air sound?
Yes I took it down to a shop and had them scan it thinking well maybe whatever it is, its just not something that gives a code, but nothing, not even anything pending, only prior codes are for the ABS module in the rear.

No air sound when removing the fuel cap.

I don't have a fuel pressure gauge handy, what should this one run? I can get a gauge on Monday to check pressure...only issue is should I check the pressure after I have driven it like normal when the problem occurs, maybe I should check cold, then drive it shut it off and restart it then check again?

Exhaust back pressure test? Test for a possible plugged cat I assume? This thought has also crossed my mind as being a partially plugged cat but I'd think that would be occurring continuously and it would cause a severe drop in acceleration that wouldn't go away, at least that's what happened with my pickup, it just wouldn't go any faster than about 30mph LOL.
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post #4 of 22 Old 01-25-2020, 06:05 PM
Wolfeman1992
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The sound your hearing might be backfiring from the intake from a lean/rich fuel condition

94 ZJ 4.0 4.5" on 31s
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post #5 of 22 Old 01-25-2020, 06:10 PM Thread Starter
wildbill23c
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Originally Posted by Wolfeman1992 View Post
The sound your hearing might be backfiring from the intake from a lean/rich fuel condition

94 ZJ 4.0 4.5" on 31s
Its not very loud so possible I guess, but what would be causing that?
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post #6 of 22 Old 01-25-2020, 06:34 PM
Wolfeman1992
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Allot actually bad o2 sensor, cracked exhaust before the o2, clogged cat, stuck or clogged fuel injectors, bad fuel pump, bad fuel feed check valve, bad pressure regulator, clogged return line or fuel filter, hell you could have a electrical problem causing the injectors to fire too much or not at all a bad relay, the list is pretty life extensive

Seeing as you only have problems when the engine is hot id say is too rich problem so that rules out some stuff but still leaves allot

94 ZJ 4.0 4.5" on 31s
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post #7 of 22 Old 01-25-2020, 06:41 PM Thread Starter
wildbill23c
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Quote:
Originally Posted by Wolfeman1992 View Post
Allot actually bad o2 sensor, cracked exhaust before the o2, clogged cat, stuck or clogged fuel injectors, bad fuel pump, bad fuel feed check valve, bad pressure regulator, clogged return line or fuel filter, hell you could have a electrical problem causing the injectors to fire too much or not at all a bad relay, the list is pretty life extensive

Seeing as you only have problems when the engine is hot id say is too rich problem so that rules out some stuff but still leaves allot

94 ZJ 4.0 4.5" on 31s
Thanks, I guess I may have to have a shop do some poking around and see what they can figure out they got all the fancy testing stuff LOL. After I attempted a head gasket repair on a car last year I won't dig into anything anymore other than regular maintenance items LOL.
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post #8 of 22 Old 01-25-2020, 07:02 PM
Wolfeman1992
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Lol try a water pump on a dodge stratus, its fwd and the pumps under the timing

94 ZJ 4.0 4.5" on 31s
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post #9 of 22 Old 01-25-2020, 07:36 PM
RedRiverT
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"Never does it when its cold...after it has ran a while, then shut it off get back in it and drive away you go to accelerate and it just won't accelerate no matter how hard you press on the accelerator"

Mine started doing that several years ago and it turned out to be a fuel pressure problem. Ever so often the gauge would drop and flutter, then come back up. I changed the pressure regulator, but if I did it again, would probably change the whole pump module. Mine didn't have the shudder though, I don't think, and definitely didn't have a knock. Mine would drive fine to say, the grocery, 12 miles or so, but then when I pulled out of the parking lot to leave, it wouldn't accelerate. I would let off the gas and then it would drive fine. It seemed to do it at the grocery or maybe a big Walmart where I idled along for several hundred yards through the parking lot before pulling out on the street.
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post #10 of 22 Old 01-25-2020, 08:32 PM Thread Starter
wildbill23c
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Originally Posted by RedRiverT View Post
"Never does it when its cold...after it has ran a while, then shut it off get back in it and drive away you go to accelerate and it just won't accelerate no matter how hard you press on the accelerator"

Mine started doing that several years ago and it turned out to be a fuel pressure problem. Ever so often the gauge would drop and flutter, then come back up. I changed the pressure regulator, but if I did it again, would probably change the whole pump module. Mine didn't have the shudder though, I don't think, and definitely didn't have a knock. Mine would drive fine to say, the grocery, 12 miles or so, but then when I pulled out of the parking lot to leave, it wouldn't accelerate. I would let off the gas and then it would drive fine. It seemed to do it at the grocery or maybe a big Walmart where I idled along for several hundred yards through the parking lot before pulling out on the street.

Hmm, kind of what mine is doing actually, but the shudder, kind of hard to really describe it, the harder you press the accelerator while its doing this kind of shudders, or maybe a better explanation would be the vehicle kind of bucks like you are on and off the accelerator, but its not accelerating hardly at all if at all. Sounds fuel related to me as well, and of course another case of a $1000 repair bill to drop a tank and replace a fuel pump thanks to the terrible design of most vehicles with no fuel pump access above the tank LOL.
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post #11 of 22 Old 01-27-2020, 08:48 AM
zjosh93
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There's a known issue with heat soak on the 4.0. You get fuel boiling in the rail and when you start it hot the injectors are injecting fuel vapor. It's enough to idle but not enough for acceleration. It's a bigger issue with the later models but occasionally it happens to the ZJs.

DEI makes an insulation kit: https://www.amazon.com/Design-Engine.../dp/B015NOKWQY

If you've done any fuel system work you can also get actual air bubbles in the fuel rail that make the situation worse. I've had luck hitting the Schrader valve to purge the vapor when it's hot and fixing the problem but it hasn't always helped other folks so YMMV. If you do that be careful, you're venting fuel and vapor right above hot exhaust, don't burn down your Jeep.
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post #12 of 22 Old 01-28-2020, 10:20 PM
LDWJ
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I'm thinking a chunk is maybe broken off in your cat and it temporarily plugs it up. My sons truck acted like this a couple times before it plugged completely and had no acceleration above idle. I pulled the cat off and shook broken pieces out and put it back on and it ran fine. Ordered new cat.
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post #13 of 22 Old 01-29-2020, 02:02 PM
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I have what sounds like the same problem with my 1997 Chevy K1500. It doesn't always happen, but when it does it seems that 95% of the time it's when I reverse out of a driveway or parking place, then shift to D and immediately drive away.



No noise, no sense of "bogging" or anything - just rolls along and won't accelerate except maybe just barely.



If I let up on the pedal briefly - less than a second - it runs fine. Or if I wait a second after shifting from R to D before pressing the pedal, it runs fine.


No codes, doesn't seem to matter if it's warm or cold. Just another one of those "old truck" mysteries.


Oh, and I recently upgraded from the original Vortec sequential ignition system to the new Delphi-designed multi-port injection - didn't eliminate the problem.


Not asking for a diagnosis on a non-Jeep; just want to point out that this doesn't seem to be unique to Jeeps.
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post #14 of 22 Old 01-29-2020, 05:55 PM Thread Starter
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Originally Posted by LDWJ View Post
I'm thinking a chunk is maybe broken off in your cat and it temporarily plugs it up. My sons truck acted like this a couple times before it plugged completely and had no acceleration above idle. I pulled the cat off and shook broken pieces out and put it back on and it ran fine. Ordered new cat.
I thought about something like that as well, but since its only after its warmed up a cat issue wouldn't exactly just do that when its warm and not cold, at least I wouldn't think so...the past couple of times I drove the Jeep it didn't do it at all, so its still an unsolved mystery. No fuel work has been done to it at least sense I've had it. I've also thought possibly a fuel pump issue too, and as soon as I get my hands on a fuel pressure tester I'm going test and see what that shows me.

Heat soak was my other theory, but at the same time my 96 ZJ never did this and its the same dang thing as my 98 ZJ, same engine, transmission, etc. But still thought about a possible heat soak issue, and the trouble is I have no idea when its going to do its non-acceleration bit, and when it does the vehicle is doing about 20mph then you press on the accelerator to speed up and it don't do anything it may try and speed up a little but won't do anything unless I let off the accelerator, then it'll be fine....so if it were heat soak I'd think it wouldn't even go 20mph LOL.
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post #15 of 22 Old 01-29-2020, 06:00 PM Thread Starter
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Originally Posted by Colt44 View Post
I have what sounds like the same problem with my 1997 Chevy K1500. It doesn't always happen, but when it does it seems that 95% of the time it's when I reverse out of a driveway or parking place, then shift to D and immediately drive away.



No noise, no sense of "bogging" or anything - just rolls along and won't accelerate except maybe just barely.



If I let up on the pedal briefly - less than a second - it runs fine. Or if I wait a second after shifting from R to D before pressing the pedal, it runs fine.


No codes, doesn't seem to matter if it's warm or cold. Just another one of those "old truck" mysteries.


Oh, and I recently upgraded from the original Vortec sequential ignition system to the new Delphi-designed multi-port injection - didn't eliminate the problem.


Not asking for a diagnosis on a non-Jeep; just want to point out that this doesn't seem to be unique to Jeeps.

My Jeep doesn't matter if its in drive or reverse it'll fail to accelerate, so its not a reverse/drive issue, its gotta be some sort of fuel delivery problem that presents itself when warm. Yep its an old vehicle mystery for sure LOL...my 1988 Ford Bronco 2 did something similar and it was a high pressure fuel pump failing...my Jeep only has the in tank fuel pump, and I have my suspicions that the fuel pump may be intermittently failing when its warm, or a fuel vapor & heat soak problem as other members have mentioned, but my 96 ZJ never did that and it had the same engine/transmission, so I find it odd my 98 Does it frequently but my 96 ZJ never once had that problem and it was beat on quite a bit offroad and frequently covered in mud so it wouldn't have been a surprise if my 96 ZJ did this but my 98 ZJ has been babied, maybe that's the problem I need to take my 98 ZJ offroad and beat it into submission HAHA!!!
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