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TOPIC: 1984 US L Jet will not rev past 3000 rpm

1984 US L Jet will not rev past 3000 rpm 17 Dec 2014 11:58 #17822


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I have exhausted all my possible avenues on this problem car.. I have been a mechanic for 35 years and have never been stumped like this. the list of repairs is as follows:
compression all good at 175 each
no vacuum leaks whatsoever
has new air flow sensor and injectors have been cleaned and tested
all new plugs and wires, cap and rotor
new temp sensor
new fuel pressure regulators and damper
new green wire
both fuel and ignition controllers substituted for good known units
all grounds repaired and electrical continuity tests ok
cam timing is good
exhaust has been removed for testing
Any help would be greatly appreciated
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1984 US L Jet will not rev past 3000 rpm 20 Dec 2014 11:44 #17844


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Maybe a little more info. Will not rev, means not at all or stumbles or if you ease it it will eventually go higher etc. I see replaced parts but have you been able to test the actual flow of Elec and fuel etc? Timing was checked, possibly an issue with the ECU as well. I have an 83 L-jet but never experienced something like this. Kind sounds electrical as long as you do not have any fuel flow issue. Maybe someone else has some experience and will chime in.

If you post some actual troubleshooting steps you have tried and those results as well, it may be easier for some to comment.
1983 928S - 16v 4.7L L-Jetronic
Dark Grey w/ Blue Leather
85/86 exhaust manifolds
Y-Pipe
SS Hi-perf Cat
S4 springs
Bilstein shocks
Air pump delete
VIN: WP0JB0926DS861180
- - Isn't Life great? - -
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1984 US L Jet will not rev past 3000 rpm 05 Jan 2015 07:53 #17907


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pzlotoff,

This looks like a tough problem. I have discovered that sometimes going back to basics helps me to solve a tough problem.

Some questions:
- Have you solved the problem? If so, would you share the solution with the Forum?
- Did the problem start suddenly, or gradually, or after some work had be done on the car?
- Does the car run perfectly, but just hits a wall at 3000?
- What happens at 3000? Gradually stops pulling? Hits a wall, but continues to run well at 3000? Starts cutting out and running roughly?
- Did any of the work done on the car make any difference?

Thanks,
Wally
Wally Plumley
928 Specialists
www.928gt.com
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1984 US L Jet will not rev past 3000 rpm 05 Jan 2015 09:06 #17908


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The problem still exists. It started suddenly when starting last spring. The car has a slight rough idle and falls on it's face at 3000 rpm cutting out and misfiring. None of the work on the car has helped the problem. It is a frustrating problem to diagnose when all tests pass.
Thank you for any additional help.
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1984 US L Jet will not rev past 3000 rpm 05 Jan 2015 11:12 #17910


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My first suspicion would be the green wire. There was a bad bunch from Porsche a couple of years back, with reversed connections. You might want to discuss this with your vendor for the green wire, and see if they think that there would be a chance you have a bad one.

Wally Plumley
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Wally Plumley
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www.928gt.com
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1984 US L Jet will not rev past 3000 rpm 10 Jan 2015 19:38 #17918


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Sorry your car is giving you such a hard time.
the way you are discribing the problem it sounds like it could be a timing issue.
I think that falls into testing a spare EZK-ecm
Its either fuel or spark right ... and since it happened all of a sudden I would guess a cracked circuit board in the controller

see is someone here has a spare they can send you to try.
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1984 US L Jet will not rev past 3000 rpm 21 Jan 2015 08:39 #17943


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This car is a 1983 US L-jet which uses a transistorized spark controller on the inside of the right inner fender well which has been substituted along with the L jet controller to no avail. I am leaning towards ignition but everything is new except the distributor itself.
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1984 US L Jet will not rev past 3000 rpm 23 Jan 2015 07:56 #17954


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"New" does not necessarily equal "good". A "new" green wire from the bad batch will still cause problems. If you still have the old one, try subbing it back in.

There are some less-common possibilities.

The distributor controls ignition timing on the earlier cars. Have you checked the timing per Page 28-29 of the FWM? (Vacuum off, 20 BTDC @3000, full vacuum adds 13 - 17 degrees)

From Page 28-2 of the FWM, "The fuel pump relay is designed to turn off the fuel pumps when engine reaches a speed of approx. 6300 rpm." I suppose that it might be possible for a malfunctioning relay to cut the pumps off earlier.

Is there any possibility that your car has had a '78 or '79 distributor rotor installed? These models had a rev limiter built into the rotor.
Wally Plumley
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