Jeep Grand Cherokee bucking at 3000RPM. - JeepForum.com
 
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post #1 of 6 Old 07-19-2019, 11:58 AM Thread Starter
JeffSearls
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Jeep Grand Cherokee bucking at 3000RPM.

To all my mechanic friends, I need some help with a 2005 jeep grand Cherokee, 3.7L. To begin, there are no codes being registered by the car. No "check engine light". At 3000 RPM the motor "bucks", almost like the fuel is cutting out, but then comes back, then bucks again, then comes back..... This happens while driving as well as in park, but only at around 3000 RPM. Fuel pressure is a solid 58 psi when the motor is running and rev-ing, even when "bucking. Ignition coils appear to be working as they should. Vacuum is normal. Fuel pump and filter are in the tank so there's no easy access. No rough idle. No issues at lower RPM's. Symptoms are being experienced while car is in Park so I know it's not the transmission. Any help would appreciated.

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post #2 of 6 Old 07-19-2019, 03:23 PM
jtec
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how do you know there are no codes?

any recent work, repairs that may offer a clue?
You checked vacuum - try holding engine @ 1500 RPM observe vacuum - did it start to drop? indication of an exhaust restriction.

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 6 Old 07-19-2019, 07:23 PM Thread Starter
JeffSearls
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Quote:
Originally Posted by jtec View Post
how do you know there are no codes?

any recent work, repairs that may offer a clue?
You checked vacuum - try holding engine @ 1500 RPM observe vacuum - did it start to drop? indication of an exhaust restriction.
No codes showing on dash (check engine light). There may be codes that my OBD2 scanner doesn't read.
I was working on it tonight and reving it to test some of my troubleshooting, and realized it won't go over 4000RPM's. It chokes/bucks out and won't go over 4000RPM's.
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post #4 of 6 Old 07-19-2019, 07:25 PM Thread Starter
JeffSearls
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Quote:
Originally Posted by jtec View Post
how do you know there are no codes?

any recent work, repairs that may offer a clue?
You checked vacuum - try holding engine @ 1500 RPM observe vacuum - did it start to drop? indication of an exhaust restriction.
We replaced the radiator a few weeks ago. But didn't do anything towards the motor.
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post #5 of 6 Old 07-19-2019, 09:24 PM
90grandoneer
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Quote:
Originally Posted by JeffSearls View Post
No codes showing on dash (check engine light). There may be codes that my OBD2 scanner doesn't read.
I was working on it tonight and reving it to test some of my troubleshooting, and realized it won't go over 4000RPM's. It chokes/bucks out and won't go over 4000RPM's.
Welcome to the forum. Under no load, out of gear, the computer (PCM) is programmed to not rev. past 4000 RPM. I believe they're actually set for 3800 RPM. It cuts fuel to stop over revving the engine. Some more advanced tuners, such as the HP Tuner, will allow the user to change this RPM limit up or down.

As said above, DTC's read with a good diagnostic scanner are a must for tracking down problems with any computerized vehicle. Can't do much without them.....except guess.

How many miles on the vehicle and when is the last time (miles ago) the spark plugs were changed? If done, what type spark plugs were installed?

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post #6 of 6 Old 07-20-2019, 06:43 AM
jtec
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are we chasing a ghost? reving engine in P/N varoom varoom.

Lets look at -
What happens when driving vehicle - idle to say 3000 RPM ?

Then what driving say and going WOT for passing 3000 rpm + ?

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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