Throttle Pot problem

Throttle Pot problem

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Mike Hawthorn

Original Poster:

87 posts

228 months

Friday 5th March 2021
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Please forgive me for this long winded question …..... But its been a long day!!

Hoping for some advice regarding my 1996 4.2 Cerb Throttle Pots. For sometime I've suffered with the uneven running and shunting at light throttle that I understand the 4.2 can suffer from. I replaced the plugs and leads with quality items along with a bespoke rose jointed throttle link bar to minimise any slop a few weeks ago and while it did make the car more “crisp” nothing changed regards the shunting under light throttle inputs.

Yesterday I replaced both Lambda sensors and both Throttle Pots. I know this seems a little OTT but sometimes its helpful to start on a level playing field.

Now this is where it starts to get “Challenging” I set up the replaced TP's in the normal way as set out many times on this forum. Once done the car starts easily but is running a bit rough. I have EvoOlli`s MBEtool (thanks EvoOlli)running on the roof and in addition to this I have a matched pair of Vac gauges connected to each bank via the purge pipe outlets.

The vac gauges are really helpful in getting a quick balance and a few turns on the link bar and the engine smooths out nicely and I drop the tick over down to 1100 rpm “ish” however, its not perfect and I fiddle until my fingers bleed but just can't get the TP right or the adaptive numbers within limits indeed the Even bank is 30+ constantly however, I know its not flowing extra air! I double check this with a flow meter on each individual inlet and all 8 are even.

I start to suspect my Even side TP as defective. It sets up fine with the engine off and the ignition on and will appear to operate smoothly on the screen however` when the engine starts no amount of adjusting the link bar is followed by changes to the Even TP value unless a wild input is made by which time its obvious the Odd / Even balance is miles out.

I therefore fitted the original TP, set it all up but the same issue was present. At this point I pulled the plug off the Even side TP and the revs increased by around 300 revs a quick fiddle with the link rod to balance the air flow and the engine settled down and all the settings settled within limits, not perfect but OK. The TP values were identical as expected.

I then replaced the Even side TP connector and the Revs dropped by 300 and the readings went back to weird! A quick adjustment to up the tick over and the engine remains running but not happy. I then removed the TP connector on the Odd bank and the engine cuts out instantly. This indicates to me that the Even bank TP is not reading (although not throwing up any faults until I disconnected it)

I went through the whole procedure again with another TP and the same results. I can only presume the new TP's are OK but the wiring or something is at fault.

So the question is has this happened to anyone else out there? Have I missed something? Are there any joints / Connectors to check before I start pulling out wires.

Any advise will be greatly appreciated by me and my hair line.

CerbWill

670 posts

117 months

Friday 5th March 2021
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Genuine NTK lambdas? Are they working properly, switching back and forth? Where did you get the TPs from?

gruffalo

7,509 posts

225 months

Friday 5th March 2021
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First things first you have impeccable taste in cars, love your garage!!

Is your exhaust leaking, this will give duff Lambda and adaptive readings?

The manifolds are prone to cracking in these cars.


Edited by gruffalo on Friday 5th March 08:29


Edited by gruffalo on Friday 5th March 08:35

spitfire4v8

3,990 posts

180 months

Friday 5th March 2021
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Your post seems to indicate that with either of the two throttle pot 2s disconnected (that you have tried) the engine runs happily, with both adaptive trims at sensible levels (with airflows balanced across the banks) .. If that is indeed so it's not a lambda, airflow, air leak or any other problem other than either a wiring fault or an ecu fault.

WIL35

525 posts

209 months

Friday 5th March 2021
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Are you getting a Battery Fault? Mine suffered from 'the uneven running and shunting at light throttle' and after messing around with an airflow meter, new throttle pots etc, it turned out to be the alternator that needed changing. It wasn't giving the ECU enough voltage to work properly.

Mike Hawthorn

Original Poster:

87 posts

228 months

Friday 5th March 2021
quotequote all
Thanks everyone for getting back to me. The Lambdas are not NTK but quality items (can't remember the make) I'll check later. The exhaust seems OK although it has been repaired in the past but I can't see or feel any leaks. I did once get a battery fault show up some time ago but has been fine since.

It was late last night when I wrote this topic so just to confirm about the Throttle Pots If I remove the TP plug from the Even bank the revs rise and the TP readings match on the MBEtool screen as expected and all is OK and within limits.

If I replace the Even bank TP plug the revs drop and the values on screen go out of tolerance

If I now remove the Odd side TP the engine stalls. This in my mind indicates a problem with the plug and or wiring possibly ECU I was hoping to be wrong!

Just an after thought, the new TP's I fitted are not Ford with gold coloured fittings.

Finally, Thanks Gruffalo for comments on my garage. It's taken a lifetime and a very supportive wife.

spitfire4v8

3,990 posts

180 months

Friday 5th March 2021
quotequote all
The odd bank Tpot governs the fuelling for the odd bank, and the ignition for both banks .. the even throttle pot just governs the fuelling for the even bank .. so what you have described by disconnecting pot 1 and it stalls is all ok. Disconnecting pot 2 just forces the ecu to use the value from pot 1 and everything will keep running.

For years I used the "disconnect pot 2" method to set the engines up .. By forcing the ecu to use pot 1 for both banks you know that the exact same map points are being used by both banks, so if you alter the link rod to even out the adaptives the assumption is that you the have even average airflow across the engine.
This doesn't account for small injector variations amongst other things, and is why I don't use this method any more but it does work well for a quick setup once you get your head around what's going on, SO ..

if you disconnect pot 2 .. then set the link rod so that both banks are adapting by about the same amount, then you have pretty much balanced the average airflows (same fuel map position plus same adaptive values across the banks must mean same airflows give or take a smidge)

If you reconnect pot 2 and set it to the exact same percentage as pot 1 then the engine should run exactly the same as when the engine was run with tpot2 was disconnected.

If your tickover is around 1100rpm then on a 4.2 you would want to be using around 19/20 percent on the tpot values... roughly working out to around 17/18 percent at a tickover just inder 1000rpm

Edited by spitfire4v8 on Friday 5th March 15:29

Mike Hawthorn

Original Poster:

87 posts

228 months

Friday 5th March 2021
quotequote all
Hi Spitfire4v8
Thanks for this information it will help me drill down to, I hope the root cause. I was under the impression the engine would run with either of the TP's unplugged individually but not both unplugged together. I assumed the good TP would be transposed to the failed side whichever side had failed. If this is not the case because the ignition requires the odd side plugged in unplugging it would clearly stall the engine.

Thanks for you time

Mike

FarmyardPants

4,099 posts

217 months

Friday 5th March 2021
quotequote all
Unplugging the odd TP stops the engine on mine. Ironically the odd one tends to have the noisier signal of the two, which is why I wanted it to run on just the even one.

Byker28i

58,831 posts

216 months

Saturday 6th March 2021
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The other point to note when replacing the throttle link bar on the 4.2, is that unless you use some standoff spacing on the nearside where it connects, it fouls on the fuel injector rail? (or something) then you can't get the revs below 1100, no matter how hard you try as it just can't close enough.


Mr Cerbera

5,031 posts

229 months

Sunday 7th March 2021
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spitfire4v8 said:
The odd bank Tpot governs the fuelling for the odd bank, and the ignition for both banks .. the even throttle pot just governs the fuelling for the even bank .. so what you have described by disconnecting pot 1 and it stalls is all ok. Disconnecting pot 2 just forces the ecu to use the value from pot 1 and everything will keep running.
Thanks for that definition, Joolz,

I was also under the (false) impression that the removal of either ThrotPot would be compensated for by the remaining one.

Now this happens to nicely dovetail into my current prob.

Before BlackBeauty© was enforced into her current 'resting phase' I had some backfiring coming from 4 & 6 on "just off throttle" and occasionally the milf light would flick on.

During The Setvice I have replaced everything, Plugs, Leads, Coils & Lambdas and, low and behold, when I've started her in the garage, the problem comes back as soon as she starts to warm up.

Soooooo, after the advice above, I restarted her and ran her today with the even ThrotPot disconnected.

Good news: Misfire has disappeared.
Bad News: milf is on almost permanently.

My first thought is that I should expect the milf light to be on when such an important sensor unit is disconnected
BUT
I don't want to die of ignorance so could you confirm that this IS what I should expect.
It's just that no-one's mentioned it above scratchchin

Ta !
(Sorry to be long and boring)

notaping

253 posts

70 months

Sunday 7th March 2021
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I know these cars are really sexy Paul, but do you honestly have a MILF light fitted to yours. What a brilliant idea :-)

PS. MR C's a much better pseudonym. Anyone who says otherwise is just jealous they didn't think of it first.

Gordon

Mr Cerbera

5,031 posts

229 months

Sunday 7th March 2021
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Byker28i said:
The other point to note when replacing the throttle link bar on the 4.2, is that unless you use some standoff spacing on the nearside where it connects, it fouls on the fuel injector rail? (or something) then you can't get the revs below 1100, no matter how hard you try as it just can't close enough.
It's at times like these that make me believe in something, other than mankind, has an influence upon our paths in life greater than we perceive or would like to admit to....

I have taken the throttle linkage apart a few times in the last 15 years
BUT
This time an interference fit wouldn't allow me to reassemble in the same way so I put the linkage in front on the even side and behind on the odd.
The piston/block offset has never dawned of my conscious mind when doing this job before and have
never heard of anyone having to do it, so I presumed that I'd reassembled the even-sided Throttle Bodies incorrectly.

Whadaya think ?


CerbWill

670 posts

117 months

Sunday 7th March 2021
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notaping said:
I know these cars are really sexy Paul, but do you honestly have a MILF light fitted to yours. What a brilliant idea :-)

PS. MR C's a much better pseudonym. Anyone who says otherwise is just jealous they didn't think of it first.

Gordon
What we've learned here is that MrCerbera is having problems with his MILF. biggrinbiggrinbiggrin. I would attempt some advice but Jools is here so I'll apply my usual workplace maxim of 'it is better to remain silent and be thought an idiot than speak and remove all doubt' and leave it to the expert.