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Unread 01-14-2014, 06:35 PM   #1
ryanfuscozj
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1996 ZJ 
 
Join Date: Apr 2013
Location: Jacksonville, Florida
Posts: 56
4.0 ZJ Weird RPM drop

Hello all,
This has been an ongoing problem for a few months now and I think it is a vacuum leak but have read that it could be a torque converter issue. When the engine is at normal operating temp. and I am driving through the neighborhood at 25 mph, barely on the gas, the RPM's drop about 500 and go back up to normal every 2 seconds. Also when I am accelerating the same thing happens, the RPMs drop about 500 and go back up until I completely let off the gas. I feel like it is a vacuum leak but I have checked them all and they are secure and not cracking or anything. Thank you in advance.

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Unread 02-03-2014, 06:57 AM   #2
GalarneauWilliam
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1997 ZJ 
 
Join Date: Oct 2013
Location: Muskegon, MI
Posts: 111
Quote:
Originally Posted by ryanfuscozj View Post
Hello all,
This has been an ongoing problem for a few months now and I think it is a vacuum leak but have read that it could be a torque converter issue. When the engine is at normal operating temp. and I am driving through the neighborhood at 25 mph, barely on the gas, the RPM's drop about 500 and go back up to normal every 2 seconds. Also when I am accelerating the same thing happens, the RPMs drop about 500 and go back up until I completely let off the gas. I feel like it is a vacuum leak but I have checked them all and they are secure and not cracking or anything. Thank you in advance.
I had a similar issue once I found it to be the Throttle Position Sensor. They are about $20 last time I bought one. FYI the bolts holding it in have loctite on them so you may have to take the throttle body off to get it out. It's not that bad.
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Unread 02-04-2014, 01:38 PM   #3
ryanfuscozj
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1996 ZJ 
 
Join Date: Apr 2013
Location: Jacksonville, Florida
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Quote:
Originally Posted by galarneauwillia View Post
I had a similar issue once I found it to be the Throttle Position Sensor. They are about $20 last time I bought one. FYI the bolts holding it in have loctite on them so you may have to take the throttle body off to get it out. It's not that bad.
Finally a response! I appreciate it, I'll go give that a try.

EDIT: I installed a new TPS and unfortunately no luck ....
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Unread 02-06-2014, 09:23 AM   #4
GalarneauWilliam
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Is your ZJ throwing any error codes?
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Unread 02-06-2014, 12:14 PM   #5
JasonStebbins
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Sounds like a faulty sensor or power draw from something. Hard to say. What have you replaced maintenance-wise in the last few months?
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Unread 02-08-2014, 12:54 PM   #6
ryanfuscozj
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Location: Jacksonville, Florida
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Originally Posted by JasonStebbins View Post
Sounds like a faulty sensor or power draw from something. Hard to say. What have you replaced maintenance-wise in the last few months?
Oh just about everything. Plugs, wires, cap, rotor, distributor, fuel filter, oil change, oil filter, air filter, cleaned TB, TPS, cleaned IAC. I feel like it has to be a fuel delivery problem since its only when I get on the gas.
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Unread 02-08-2014, 05:48 PM   #7
JasonStebbins
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Have you checked fuel pressure at the rail? Should be 47ish while running. Try unplugging the upstream oxygen sensor and going for a short drive?

Could definitely be a vacuum leak, have you checked all the lines in the bay?
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Unread 02-08-2014, 07:27 PM   #8
ryanfuscozj
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Location: Jacksonville, Florida
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Originally Posted by JasonStebbins View Post
Have you checked fuel pressure at the rail? Should be 47ish while running. Try unplugging the upstream oxygen sensor and going for a short drive?

Could definitely be a vacuum leak, have you checked all the lines in the bay?
That was the first thing I checked but nope no vacuum leaks whatsoever. I have not checked the fuel pressure. I do have a fuel pressure gauge from a leftover turbo build, if I hook that up to the fuel rail would that work just as well? CEL came on so I'll see what that code is tomorrow.
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Unread 02-08-2014, 07:43 PM   #9
JasonStebbins
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It's just a schrader valve on the rail, so the gauge should just screw on.

Ah. That's good. Now you'll know what's wrong hopefully.
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Unread 02-12-2014, 10:56 AM   #10
ryanfuscozj
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Quote:
Originally Posted by JasonStebbins View Post
It's just a schrader valve on the rail, so the gauge should just screw on.

Ah. That's good. Now you'll know what's wrong hopefully.
Well I had the CEL checked and it was a bad upstream o2 sensor. I replaced it and the RPM's don't drop as much as they did when I accelerated so it certainly helped a little. No more CEL. Still not sure why it is doing this
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Unread 02-12-2014, 11:03 AM   #11
JasonStebbins
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Quote:
Originally Posted by ryanfuscozj View Post
Well I had the CEL checked and it was a bad upstream o2 sensor. I replaced it and the RPM's don't drop as much as they did when I accelerated so it certainly helped a little. No more CEL. Still not sure why it is doing this
Did you use a Mopar/NGK sensor or aftermarket? They are hit or miss. Unplug the battery after you did the sensor?
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Unread 02-12-2014, 11:13 AM   #12
ryanfuscozj
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Originally Posted by JasonStebbins View Post
Did you use a Mopar/NGK sensor or aftermarket? They are hit or miss. Unplug the battery after you did the sensor?
I bought a Bosch one from O'Reilly's. Yep, unplugged it for about 5 minutes.
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Unread 02-12-2014, 11:26 AM   #13
JasonStebbins
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Quote:
Originally Posted by ryanfuscozj View Post
I bought a Bosch one from O'Reilly's. Yep, unplugged it for about 5 minutes.
Bosch rarely works with our Jeeps. It's a resistance issue.
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Unread 02-12-2014, 11:30 AM   #14
ryanfuscozj
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Originally Posted by JasonStebbins View Post
Bosch rarely works with our Jeeps. It's a resistance issue.
What brand would you recommend? I wouldn't mind getting my $60 back for this o2 sensor....
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Unread 02-12-2014, 11:34 AM   #15
JasonStebbins
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Originally Posted by ryanfuscozj View Post
What brand would you recommend? I wouldn't mind getting my $60 back for this o2 sensor....
Mopar/NGK are what you will want to get. They are OEM. Most parts stores carry NGK, otherwise RockAuto has them for dirt cheap. That's where I got mine.

This goes for almost all sensors on a ZJ by the way.
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