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Unread 09-28-2014, 08:32 AM   #1
MrRoundel
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Worsening lack of "oomph" 180K 4.0L.

Greetings folks,

My 4.0L ZJ seems to be losing what little "oomph" that it has had, especially when pulling moderate hills. I always lock out overdrive, but where I used to pull at 50+, I now can barely hold 45 mph, and this is with a small payload.

I have read posts where TPS's and CPS's may cause this, or perhaps a failing fuel pump? If either of the sensors were bad, wouldn't the OBD II throw a code? And, my experience with fuel pumps seems to be either they work or they don't. Perhaps there's an internal fuel-pressure regulator? I remember that this went bad on an '84 Bimmer 533i that I had, and when it failed the car would bog to stall very quickly. I could hardly get out of my driveway. My Jeep is just a bit more gutless, doesn't stall. If you were me, what would you look into first?

I'll be irritated if it's the fuel pump, as I just put a factory skid-plate on the gas tank, thereby making the job a bit more difficult, especially for a one-man job.

Note: Front O2 sensor was replaced a few years back. Catalytic converter is original (Culprit?).

'98 ZJ Laredo 4x4, 4.0L, Selectrac/242 TC, 180,300 miles.

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Unread 09-28-2014, 09:05 AM   #2
underpowered
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catalytic converter could cause issues such as this by being clogged and restricting flow. I'd Check fuel presure while driving first before i hacked the converter off.
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Unread 09-28-2014, 09:15 AM   #3
MrRoundel
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Thanks, I'll pull out my factory manual and see where to hook up the gauge. Then I'll go to work on finding one. I may even have one, a vacuum gauge that claims it can also check fuel pressure. I think that I used it on my Bimmer years ago, and might have to put a longer vacuum/fuel line on it to be able to do the drive-vu test. Time to check the toolbox. Cheers.

Update: It looks like the tool I have in my box will not work for this application. I guess I'll have to look further.

Last edited by MrRoundel; 09-28-2014 at 09:40 AM.. Reason: Update after checking tool
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Unread 09-28-2014, 09:51 AM   #4
mikem350
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Don't think the vac gauge will read 50 psi pressure...but harbor freight ones are cheap and ok.

How about clogged air filter/MAF/airbox inlet?
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Unread 09-28-2014, 11:18 AM   #5
ZeeJay1997
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you can check cat with vac gauge. see scenario 14
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Unread 09-28-2014, 11:22 AM   #6
coralman
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I'd run a compression check while I was at it.
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Unread 09-28-2014, 06:34 PM   #7
MrRoundel
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Quote:
Originally Posted by ZeeJay1997 View Post
you can check cat with vac gauge. see scenario 14
Thanks. I've got that exact same vacuum gauge. I'll give it a shot.

Shot given: 16" vacuum at idle, goes up smoothly to around 3,000 rpm. Is 16 a normal reading for a 4.0L in a '98 ZJ?

I bought the Fuel Injection Pump Test from Harbor Freight and will try it out tomorrow.

@coralman: Good suggestion. I guess I haven't tested compression in a while. I'll have to retrieve my gauge from the last borrower.

Fuel pressure at schrader valve on rail shows steady 46 lbs at idle. Is that close enough to 49 lbs.?
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Unread 09-28-2014, 06:46 PM   #8
ZeeJay1997
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16 is about normal at idle, but the question is does the needle drop when you slowly increase the rpms. if the cat is plugged the exhaust builds up in the engine and causes low vacuum.
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Unread 09-28-2014, 07:06 PM   #9
MrRoundel
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Quote:
Originally Posted by ZeeJay1997 View Post
16 is about normal at idle, but the question is does the needle drop when you slowly increase the rpms. if the cat is plugged the exhaust builds up in the engine and causes low vacuum.
I slowly brought the rpm's up to 3000, and it climbed to around 20" rather than dropping. Is there a rule for how high to take it before it will show? Thanks.

BTW: I checked my factory manual, and it shows that the fuel pressure of 49lbs is +/- 5 lbs, so I should be ok, at least at idle. I took the rpms up a bit as well, and it stayed steady at 46 lbs.

Oh, and ZeeJay, thanks for the link. That's a great page for reading the vacuum gauge. I'll put it in my keepers, and hope they stay up and running.

Last edited by MrRoundel; 09-28-2014 at 07:44 PM.. Reason: Added appreciation to ZeeJay
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Unread 09-28-2014, 08:04 PM   #10
ZeeJay1997
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Quote:
Originally Posted by MrRoundel View Post
I slowly brought the rpm's up to 3000, and it climbed to around 20" rather than dropping. Is there a rule for how high to take it before it will show?
no hard fast rules. the cat can have varying degrees of blockage. if you're not getting any misfires or knocking when you put your foot in it, most likely the cat is not badly blocked. a rise in the engine temp will also occur with a badly blocked cat. there are some variables involved like exhaust leaks forward of the cat where blocked exhaust gases can escape instead of choking the engine.

one thing you can do just for peice of mind is remove the o2 sensor so any trapped exhaust pressure can escape. if you get an improvement in power, there's your sign.

it's not likely a probelm since the vac is not dropping, but as explained, there are variables.
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Unread 09-28-2014, 10:04 PM   #11
MrRoundel
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Quote:
Originally Posted by ZeeJay1997 View Post
if you're not getting any misfires or knocking when you put your foot in it, most likely the cat is not badly blocked.
I'll have to try that, since I rarely actually put my foot in it. I drive my cars really easy, perhaps too easy. My complaint of a lack of "oomph" is triggered by the minimal acceleration I get on moderate hills while gradually giving it more gas. Maybe I'm not cueing the transmission to downshift or something? I'll goose her a bit on the hills next time I drive the canyon, see how she behaves. Thanks again.
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Unread 09-29-2014, 05:43 PM   #12
ZeeJay1997
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engines need the soot blown out them every now and then. try some top end cleaner in the fuel tank.
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Unread 09-29-2014, 06:07 PM   #13
MrRoundel
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Originally Posted by ZeeJay1997 View Post
engines need the soot blown out them every now and then. try some top end cleaner in the fuel tank.
Is that the same as fuel injector cleaner, like Techron? I was just about to run a bottle in my next tank. Thanks.

Another thing I just noticed is it looks like I haven't changed my plugs for over 60K! A bit excessive maybe? That's very odd, but I do take care of a few cars. There has been an occasion or two where I've forgotten to note a service, and this may have been another of them. Then again, it may have been over 60K miles since I last changed plugs. I think the owner's manual calls for 30K intervals. I ordered some NGK FR51's (V-Power, FWIW). I've had good luck with NGK's over the years. I went with free shipping so it's going to take until the middle of next week to get them.
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Unread 09-29-2014, 06:47 PM   #14
jlann
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Quote:
Originally Posted by MrRoundel View Post
I slowly brought the rpm's up to 3000, and it climbed to around 20" rather than dropping. Is there a rule for how high to take it before it will show? Thanks.

BTW: I checked my factory manual, and it shows that the fuel pressure of 49lbs is +/- 5 lbs, so I should be ok, at least at idle. I took the rpms up a bit as well, and it stayed steady at 46 lbs.

Oh, and ZeeJay, thanks for the link. That's a great page for reading the vacuum gauge. I'll put it in my keepers, and hope they stay up and running.
Where did you attach the gauge at?
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Unread 09-29-2014, 06:56 PM   #15
ZeeJay1997
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i've never used techron but i've seen others swear by it. i've always used lucas.

if the plugs have 60k, you may want to look at the wires. order a set of denso original fit 671-6128
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