Intermittent Acceleration Failure ONLY from Stop -- 97 D90 - Defender Source
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  #1  
Old March 17th, 2009, 01:18 PM
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Thumbs up Intermittent Acceleration Failure ONLY from Stop -- 97 D90

I have intermittent problems with acceleration from a stopped position. It happens at about every other stop. I first noticed the issue during an offroading trip. I have searched a variety of threads, but others seem to have similar issues under normal/high speed driving conditions.



Here’s how it feels:



From a stop, apply throttle, begins to accelerate for split a second, drops out completely for a second, then surges to catch up.

  • Only happens when the truck is stopped – never while cruising at speed.
  • Happens when engine is hot or cold
  • Happens at full or nearly empty tank
  • Since first noticing it, I have replaced the MAF sensor and both Cats (for unrelated codes thrown), and neither has had an effect.
  • I currently have no codes or warning lights on the system
  • Plugs and wires are less than 1 year old.
  • Fuel filter is less than 1 year old.
Possibly related issues?

Alternator was changed out about 3 months prior to first noticing problem.



Any ideas appreciated... Thanks!
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  #2  
Old March 17th, 2009, 02:31 PM
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Sounds like a throttle position sensor to me.

Eric
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  #3  
Old March 17th, 2009, 02:35 PM
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x2 to what eric said.
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  #4  
Old March 17th, 2009, 03:24 PM
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Thumbs up

The Service Manual has some info on the TPS (below). My MIL has not illuminated, and idle does not appear to be effected, but the other symptoms seem to be in line with your suggestion.

Oddly enough, i have noticed a slight hesitation at the 1750 rpm range in the past.

Can i assume that if the sensor is on the fritz, it would create these symptoms and never trigger the MIL?


Throttle position (TP) sensor:
The throttle position sensor is mounted on the plenum chamber and connected directly to the throttle shaft. The sensor is a variable resistor, the signal from which (0-5V) informs the ECM of the actual position of the throttle disc.As there is no default strategy, failure of the sensor will result in poor idle and lack of throttle response. If failure occurs in the closed position the engine will only reach 1750rev/min when the ECM will initiate overrun fuel cutoff. The fault is indicated by illumination of the malfunction indicator light (MIL).
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  #5  
Old March 17th, 2009, 03:28 PM
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"Can i assume that if the sensor is on the fritz, it would create these symptoms and never trigger the MIL?"

Yes. You can test the TPS with a scanner it should move linearly.
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  #6  
Old March 17th, 2009, 03:30 PM
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Mine used to do the same thing when I was on the interstate at cruising speed, the TPS had a dead spot and it would scare the crap out of me.
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  #7  
Old March 17th, 2009, 11:23 PM
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Thumbs up

Thanks guys. I'll test the sensor and see what turns up... Anyone have a spare lying around that they want to sell?
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  #8  
Old March 18th, 2009, 12:07 AM
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I vote for the TPS. For some time I would get an intermittent TPS fault in my 95 SW. I would get hesitation sometimes just like you described. then one day, the truck would barely run at all, especially with a load on the engine. that is I could accelerate the engine in neutral, but when driving it ran so bad I turned around and limped home after about a mile. I replaced the TPS and it has run perfectly smooth since.
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  #9  
Old March 18th, 2009, 08:30 AM
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Same issue for me on my Classic, TPS, my truck would cut out on the highway.
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  #10  
Old March 24th, 2009, 02:34 PM
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Not the TPS

Brand new TPS arrived last night. Installed moments ago. Problem did NOT go away.

Any other thoughts?
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  #11  
Old March 24th, 2009, 02:37 PM
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I give up.
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  #12  
Old March 24th, 2009, 02:45 PM
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Thinking....

Thinking...
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  #13  
Old March 24th, 2009, 02:48 PM
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How about cleaning the butterfly on the intake?
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  #14  
Old March 24th, 2009, 03:31 PM
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Quote:
Originally Posted by evilfij
How about cleaning the butterfly on the intake?
I agree with Ron. Check the air intake hose for holes too.

Eric
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  #15  
Old March 24th, 2009, 03:36 PM
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OK. Will do.

The stepper motor has also come up in my research on the board. That's next on the list i guess...
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  #16  
Old March 24th, 2009, 05:17 PM
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Check the vacuum hose from the Plenum to the fuel pressure reg...
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  #17  
Old March 24th, 2009, 07:10 PM
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How about a weak throttle spring or weak overrun spring?
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  #18  
Old March 24th, 2009, 08:00 PM
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How many miles on the truck? I always thought the stepper would just cause idle problems? I thought a throttle spring would only close the throttle. Does it do it at WOT?
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  #19  
Old March 24th, 2009, 08:04 PM
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32,000 miles on the truck.

The problem is only at the opening range of the throttle when starting from a stopped position. Not at any other time.
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  #20  
Old March 24th, 2009, 09:30 PM
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Are your RPMs climbing without vehicle movement?
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