Throttle Pot Issues

[JackFitz]

So I had a couple of issues before rebuilding my engine where the throttle pot percentages would jump +/- upto 2% on the odd cylinder bank. It wasn't too much of an issue.

Now however, it's jumping +/- 5% and I'm really struggling to get the engine to run smoothly as I can't set it up properly. I've replaced both throttle pot sensors which has made no difference. I've just checked the wiring which is showing the same, very little to no resistance to the ECU plug on each connection so I don't think there are any breaks in the wiring.

Does anyone have any further suggestions

Thanks!

[JackFitz]

Just tried soldering a new wire from pin 35 on the ECU plug to the central pin on the odd cylinder bank throttle pot connector and that has made no difference. Values are still jumping between 17 and 21%. Could it be a problem with the ECU?

[Byker28i]

Have you tested the throttle pots, are they good? Normally when they start to fail they jump around

[JackFitz]

Yeah, the throttle pots are brand new. Have tried swapping the connectors (odd to even, even to odd) and the odd bank is still showing as the problem.

[Penelope Stopit]

Suggest that you check voltages at TPS1 and TPS2, then check for similar voltages from TPS1 and TPS2 at the ECU

As you know, there should be a negative at each TPS (Pin 1) from ECU Pin 49, there should be 5 Volts positive at each TPS (Pin 3) from ECU Pin 48

Then there are the outputs from each TPS (Pin 2), measure these outputs through a sweep of the throttle

In summary, check voltages at each TPS and then check voltages at the ECU

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Don't bother with resistance values for checking the wiring, wiring needs a load on it for proper testing......

Disconnect ECU and Both TPS before the below test is carried out

Earth each cable in turn at each TPS plug while connecting a 21 watt bulb from battery positive to the other end of each corresponding cable at the ECU Plug

Check the bulb is fully illuminated and measure the voltage from battery positive to each cable at the ECU plug while the 21 watt load is being applied

Disconnect ECU and Both TPS before the above test is carried out

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If you have no idea what I'm on about....................Ask

[JackFitz]

Thanks for that.

I checked the voltages for both tps1 and tps2 on pin 18 and 35 at the ecu and these were both reading 1.02v and increased as the throttle was opened. I checked pin 3 on both tps connections and these were varying between 5.3v and 5.5v.

I have just taken a close look at the ECU and have noticed this broken track to one of the ground pins. Going to try and solder a bridge across the gap and hope this sorts it...

[Penelope Stopit]

Mmmm, the problem is that something caused that track to burn out

[MoonMonkey]

Hi Jackfitz

Have you looked for a mechanical issue?

I had free play in my throttle spindle which kept causing one banks throttle pot to spike causing poor running on part throttle.

Took all morning with Joolz to work it out. If you put a small amount of pressure on the end of the spindle with a screw driver, the problem went away. Unfortunately a bit difficult to do while driving!

Just another idea

Thanks

[JackFitz]

Well I soldered that gap up with a .wire but it's made no difference. I'll check for play in the throttle but the values are jumping even when the engine isn't running.

[spitfire4v8]

Ive tried all sorts of things over the years to try and get a really stable tpot1 input but sadly nothing really works. I too think it's an ecu issue but not limited to your ecu, all the ajp8 ecus do it to some extent .. interestingly the later evolutions on sp6 cars seem not to. If yours is particularly bad then try borrowing someone else's ecu to see if that improves it, though what you might do to your ecu to effect a change I really don't know. You need a proper electronics man on the job.
Its rare to find it spiking enough to upset the running though .. by the time it's that bad then a change of Tpot gets you back somewhere right as it's usually the Tpot that causes the major issues (and spindle play as mentioned above) ..

Ive put ferrites around the wires (naff all effect) , reduced the throttle sensitivity in the software (just reduces throttle response which isn't good) , re-wired the ecus and Tpots separately, shielded wiring, nothing really helps

Tpot2 is never as affected though, but why that should be I have no idea.

If you ever sort it out please let us all know, it would be valuable info.

[JackFitz]

I did wonder if it was common on other cerbs. I'll try and setup the engine as it is and see. If it's no good then I'll try to find another ECU to test with. Thanks for the advice.

[Penelope Stopit]

Would be a good idea to attempt finding out what burnt that track before fitting another ECU

[spitfire4v8]

From the position in the ecu it looks like it's pin 7 which is a ecu system earth pin. No idea why that should burn out but tracing it through the pcb and seeing where the other end is might give you a clue. The car side should go to the common earth point behind the dash I think... not too sure without looking at a car but that's the earthing point for all the engine run ecu stuff. You should be able to trace that side easily enough

Edited by spitfire4v8 on Monday 7th September 13:44

[gruffalo]

JackFitz said:

I did wonder if it was common on other cerbs. I'll try and setup the engine as it is and see. If it's no good then I'll try to find another ECU to test with. Thanks for the advice.

I will have an ECU going spare in a few weeks time if you need one.

You will just have to swap the EEPROM as it is mapped for my 4.7.

[gruffalo]

JackFitz said:

I did wonder if it was common on other cerbs. I'll try and setup the engine as it is and see. If it's no good then I'll try to find another ECU to test with. Thanks for the advice.

I will have an ECU going spare in a few weeks time if you need one.

You will just have to swap the EEPROM as it is mapped for my 4.7.

[JackFitz]

gruffalo said:

I will have an ECU going spare in a few weeks time if you need one.

You will just have to swap the EEPROM as it is mapped for my 4.7.

Thanks for the offer! I managed to set the engine up and took it for a drive. It ran lovely until I heard a clatter and I was down to 7 cylinders and lost the assisted braking.

Checked tonight and there was no compression in the number 7 cylinder (explains why the assisted breaking went). The exhaust valve clearance seems to have closed right up on that cylinder and is being held partially open. Not sure why that's suddenly happened as I adjusted all of the valve clearances during the rebuild.

[ukkid35]

JackFitz said:

Thanks for the offer! I managed to set the engine up and took it for a drive. It ran lovely until I heard a clatter and I was down to 7 cylinders and lost the assisted braking.

Checked tonight and there was no compression in the number 7 cylinder (explains why the assisted breaking went). The exhaust valve clearance seems to have closed right up on that cylinder and is being held partially open. Not sure why that's suddenly happened as I adjusted all of the valve clearances during the rebuild.

My guess is that the shim has tried to escape and is now jammed between the spring retainer and the bucket, preventing the valve from fully closing

I am not entirely sure what might have caused it to happen, and thankfully it has not recurred on my engine, but if your rebuild included new guides as mine did, then that might be a clue

Thank fully it happened on 8 rather than 7 so I didn't lose braking assistance

[ukkid35]

[JackFitz]

Completely lost as to what caused that. The valve shim was still in-place. There was nothing out of the ordinary. I sanded a bit off of the shim so that the clearance was correct again, did another compression test and everything was back to normal. Put 100 miles on it yesterday without any problems so hopefully that should be fine now... Just completely lost as to what caused the valve clearance to close like that.