Map Tune Resistor Faulty ?

[neutral 3]

Hi all, my 98 is running rough, down on power, surges and holds back from around 2,000 rpm but revs cleanly when at a stand still and an ECU mate test today, says Tune resistor is faulty.

I disconnected it a couple of times and the 2 pin conector looked a bit corroded. When I then drove it its running better but still not right.
Any ideas anyone ? Car has been de catted and has the decat resistor.

[fausTVR]

Corrosion on those pins would alter the resistance being seen by the ECU, so try a good clean up first. I think connector corrosion is the root cause of a million car faults and keeps garages in business.

[EGB]

Yes as of Faust. Also treat points with electrical cleaner. Servisol or from Halfords.
Also can you tell me where the tune resistor is? and what colour. Mine is a 1999.
Thanks.......Eddie

[Hedgehopper]

I'm sure someone will correct me if I'm wrong but I believe the tune resistor is in the plastic sleeve on the left of the pic. (next to the 'other' 100 amp fuse, with its cover open).

[davep]

I think the answer is explained on page 30 of the ECUMate manual:

http://ecumate.com/docs/Ecumate%20inst.pdf

That is: if no (or possibly incorrect) resistance value detected by ECU then the limp home fuel map is selected.

Even if you don't have ECUMate the manual is worth a read.

[EGB]

Thanks Alex. Will check on mine.

[neutral 3]

Thanks for replys, Does the Tune resistor only work at lower throttle openings ?

I cleaned the contacts again and still very rough, surging and hesitating low down, it's barely drivable as it is

[davep]

neutral 3 said:

Thanks for replys, Does the Tune resistor only work at lower throttle openings ?

I cleaned the contacts again and still very rough, surging and hesitating low down, it's barely drivable as it is

The value of the tune resistor is only checked when the ECU is first switched On when the ignition is switched On. The ECU then selects the fuel map that corresponds to the tune resistor value. If the resistor value is not one of the expected values the limp home map is used. I'd check the value of the tune resistor and circuit continuity to eliminate this as a cause.

[blitzracing]

That error is very common , and should clear with an ECU reset. It seems to occur if the ECU has had the tune switched at some point with out powering the ECU off and on again with a disconnect. You need to check what map its actually running on and the ECUmate will clearly say limp home if the ECU is get you home mode. If the ECUmate says its running non cat map then the tune resistor is not your problem for poor running. The ECUmate will only report back the error codes that the ECU generates, and the ECU software had many tweaks over the years to reduce its chance of producing false errors, so you cant guarantee to get an relevant error code when its running badly, its not that sensitive. Also as you are running the non cat map about 50% of the fault code information is missing from the diagnostics as there is no lambda feedback for the ECU to pick up the fact the fueling is wrong. You will need to check the sensor readings carefully, and check for obvious things like sooted plugs, or plug variances between the 8 cylinders.

Edited by blitzracing on Sunday 7th July 21:07

[neutral 3]

I think the only way to find the fault will be via a rolling road with someone who knows these cars, any one recomend one ?

[neutral 3]

On sat I disconnected the ECU unit and sprayed the contacts with some WD and did the same with some of the other electrical relay contacts etc and what a difference.

Not sure if the contacts on the ECU were corroded or if disconnecting it has re set it ?

But another problem has now surfaced, the Stepper motor thread / or the plenum thread ? is worn and the stepper wont tighten and it's pulling in air. Anyone know what I can seal it with ?

[blitzracing]

PTFE tape if there is enough thread left for it to tighten up lightly.

[EGB]

neutral 3 said:

On sat I disconnected the ECU unit and sprayed the contacts with some WD and did the same with some of the other electrical relay contacts etc and what a difference.

Not sure if the contacts on the ECU were corroded or if disconnecting it has re set it ?

But another problem has now surfaced, the Stepper motor thread / or the plenum thread ? is worn and the stepper wont tighten and it's pulling in air. Anyone know what I can seal it with ?

Use proper electric cleaner next time, made fit for purpose. WD (Diesel?) is not IMOP.
Stepper has been overtightened in the past. A SH Bible warning.