3.9 Engine hesitation under acceleration - Defender Source
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  #1  
Old January 19th, 2015, 09:29 AM
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William
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3.9 Engine hesitation under acceleration

I have searched for this and no luck. I have a 1994 NAS 3.9 V8. Had a lot of recent work done, tune up, new plugs, wires, distributor, fuel filter, fuel pump, fuel lines, new K&N air filter and snorkel added.

Just noticed recently (about 3 months since all tune up work was done) that under about half acceleration (mid way pushing the pedal to the floor) the engine hesitates and bucks very slightly then goes away if I back off the throttle a bit.

Now I know some may say don't push the acceleration to half way and be fine but it didn't do this right after the tune up and ran really smoothly but not so much under acceleration any more.

Thoughts on this? I should mention the MAF was also taken apart, cleaned out and new o-ring put on and was check for any holes / leaks.

Is the throttle positioning sensor a possibility?

Thanks!
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  #2  
Old January 19th, 2015, 09:36 AM
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Joshua
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What did you search for without any luck? Just curious. It can be quite a few different things. I know your exhaust is new, but are your o2 sensors new as well? Is your timing correct? Are you throwing any codes? Did you ever notice it previously before the work? Does it happen under load as well, such as while going uphill?
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  #3  
Old January 19th, 2015, 10:10 AM
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William
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I never noticed it before but you may be on to something. I installed the new exhaust but the O2 sensors were not replaced at that time. I did have a check engine light for a bit but we think it was the charcoal filter which had the hose that connects to it broken. The filter was all busted up as well. Thanks I will look into the O2 route.
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  #4  
Old January 19th, 2015, 05:12 PM
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Jason England
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There is virtually zero probability that this could be O2 sensors.

You should test the resistance of the TPS and make sure it's smooth for the full swing ... any worn spots could cause the condition you describe ... however also not very likely.

Most likely is an air leak in a hose somewhere.
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  #5  
Old January 19th, 2015, 05:41 PM
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Mike Ferguson
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I had a very similar problem, but more severe, primarily when the engine was cold. The engine would idle, but would surge and die if you applied any throttle until it warmed up. Replaced the TPS and it cleared up. Easy to replace and not very expensive. Good luck.
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Old January 19th, 2015, 06:16 PM
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Quote:
Originally Posted by Angus View Post
I had a very similar problem, but more severe, primarily when the engine was cold. The engine would idle, but would surge and die if you applied any throttle until it warmed up. Replaced the TPS and it cleared up. Easy to replace and not very expensive. Good luck.
Can you send me your old TPS please?

A TPS is nearly $200 ... I would want to know it was actually faulty before replacing it.
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  #7  
Old January 19th, 2015, 06:23 PM
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Mike Ferguson
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Sorry, it was a year or so ago and I did not keep it after it proved to be the culprit. That is more expensive that I remember. I am sure someone on here has a used one they will part with. You can check your old one with a volt-ohm meter to ensure there is a smooth change in the resistance. If there are spikes of flat spots, that could be your problem.
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  #8  
Old January 19th, 2015, 09:36 PM
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All the vacuum hoses replaced as well or cleaned out at the hard points? Port on the throttle body dirty? The flame trap canister on top of the valve cover cleaned or replaced recently? Do you still have the old cats on your exhaust pipe?
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  #9  
Old January 19th, 2015, 09:42 PM
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Quote:
Originally Posted by airbornrover View Post
All the vacuum hoses replaced as well or cleaned out at the hard points? Port on the throttle body dirty? The flame trap canister on top of the valve cover cleaned or replaced recently? Do you still have the old cats on your exhaust pipe?
No to all the above except the cats are new like the exhaust.
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  #10  
Old January 19th, 2015, 10:08 PM
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Check to see if the hoses are clogged or the connector pieces are gunked up with deposits. Same goes for port on the throttle body and flametrap. Just soak the flametrap in some solvent till you buy a new one or what have you, the connectors and port you can clean with a qtip with solvent.
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  #11  
Old January 20th, 2015, 11:57 PM
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John Bradley
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Make sure your rotor is distributing the proper amount of spark. If you still have the old rotor handy, throw it on and see if this remedies the issue. You might have a defective rotor.
Cheers!
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  #12  
Old January 21st, 2015, 12:43 AM
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What are you running for fuel? Low end or high end octane?
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  #13  
Old January 22nd, 2015, 03:41 PM
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William
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Thanks all seems to have remedied itself. May have been the car was sitting for over a month in the shop and was very cold in New Hampshire when I picked it up. Seems to run smoothly again.
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