Warm idle stall

[ZX81]

Wondering if anyone can point me in the right direction for this problem...
Cold start is fine and idles low then picks up and normal
Once warm (70C+) on clutch in idle drops to 3-500 then picks back up to normal 8-900 on flat or downhill.
If uphill, idle drops even more (1-200) and wants to stall, occasionally doing so.
If switch off and restart, no issues at all and idle now behaves perfectly with no issues for the rest of the journey.
I've taken to driving down the road a bit, pulling over and switching off / back on when commuting so it doesn't stall somewhere awkward.
So I guess it seems to be ecu related mapping dropping too low when switching from cold to warm and the off/on resets it ? I'm sure there's a simple solution, just not sure where to start (Mk2 4L 1996 serp, recently serviced, running ceramic ht and iridium 6 sparks).

[spitfire4v8]

This situation often occurs when there is no road speed signal detected by the ecu.

If you can get it to a garage (or a local tvr owner) which uses rovergauge you can see if the road speed is being seen by the ecu or not.

high engine speed + zero road speed results in the ecu closing the stepper down trying to reduce the revs.

In the meantime you can disconnect the idle valve electrical connector when the engine is idling properly to stop it adjusting, you will just need to be mindful of the engine maybe wanting to stall when it is very cold, but once you've done a couple of miles it should be stable.

[ZX81]

Thanks for info spitfire.
Managed to dig out my cable and ran it, no errors with anything (including lambdas which was a problem for me the last time), however it was peculiar on the road speed where it pretty much read max or 0. See below for snippets of rover log, interestingly the lowest it detected the revs on the run was 635, but cabin gauge was definitely showing lower at times.

Anyway checked the idle value grub screw and found it was only at a half turn from end, so have opened it to 1.25 turns. On quick depress of clutch approaching lights it still drops close to zero but instantly picks back up to 900 ish and doesn't over rev. I shall continue to tweak this as I know factory is 2.5 turns, but seems mine doesn't need that much.

For those with similar problems reviewing this, picture of idle screw at the bottom.

1. datetime roadSpeed engineSpeed

2022-03-20_16:36:18 0 1601
2022-03-20_16:36:19 0 2079
2022-03-20_16:36:19 22 2094
2022-03-20_16:36:20 22 1605

2022-03-20_16:37:27 54 1285
2022-03-20_16:37:27 54 1236
2022-03-20_16:37:28 0 1264
2022-03-20_16:37:28 0 1253
2022-03-20_16:37:29 0 1308
2022-03-20_16:37:30 0 1207
2022-03-20_16:37:30 53 1791
2022-03-20_16:37:31 53 2283

2022-03-20_16:39:18 53 1275
2022-03-20_16:39:18 53 1296
2022-03-20_16:39:19 0 1147
2022-03-20_16:39:19 0 937
2022-03-20_16:39:20 0 873
2022-03-20_16:39:21 0 881
2022-03-20_16:39:21 0 892
2022-03-20_16:39:22 0 884
2022-03-20_16:39:23 0 925
2022-03-20_16:39:23 0 949
2022-03-20_16:39:24 0 973
2022-03-20_16:39:25 0 969
2022-03-20_16:39:25 0 935
2022-03-20_16:39:26 0 961
2022-03-20_16:39:27 0 945
2022-03-20_16:39:27 0 945
2022-03-20_16:39:28 0 946
2022-03-20_16:39:28 0 958
2022-03-20_16:39:29 0 971
2022-03-20_16:39:30 0 970
2022-03-20_16:39:30 0 940
2022-03-20_16:39:31 0 922
2022-03-20_16:39:32 0 922
2022-03-20_16:39:32 0 963
2022-03-20_16:39:33 0 988
2022-03-20_16:39:34 0 978
2022-03-20_16:39:34 0 941
2022-03-20_16:39:35 0 934
2022-03-20_16:39:36 0 929
2022-03-20_16:39:36 0 914
2022-03-20_16:39:37 0 902
2022-03-20_16:39:37 0 976
2022-03-20_16:39:38 0 1026
2022-03-20_16:39:39 0 1035
2022-03-20_16:39:39 0 938
2022-03-20_16:39:40 0 897
2022-03-20_16:39:41 0 866
2022-03-20_16:39:41 0 897
2022-03-20_16:39:42 0 950
2022-03-20_16:39:43 0 933
2022-03-20_16:39:43 0 990
2022-03-20_16:39:44 0 986
2022-03-20_16:39:45 0 955
2022-03-20_16:39:45 0 908
2022-03-20_16:39:46 0 905
2022-03-20_16:39:46 0 947
2022-03-20_16:39:47 0 968
2022-03-20_16:39:48 0 962
2022-03-20_16:39:48 0 1647
2022-03-20_16:39:49 0 1141
2022-03-20_16:39:50 35 1524
2022-03-20_16:39:50 35 2102
2022-03-20_16:39:51 53 2446
2022-03-20_16:39:52 53 2781
2022-03-20_16:39:52 53 2232
2022-03-20_16:39:53 53 1612
2022-03-20_16:39:54 53 1915

2022-03-20_16:43:48 51 1697
2022-03-20_16:43:49 51 1644
2022-03-20_16:43:50 53 1610
2022-03-20_16:43:50 53 1618
2022-03-20_16:43:51 53 1472
2022-03-20_16:43:52 53 795
2022-03-20_16:43:52 54 1192
2022-03-20_16:43:53 54 1644
2022-03-20_16:43:54 53 635
2022-03-20_16:43:54 53 894
2022-03-20_16:43:55 54 1623
2022-03-20_16:43:56 54 1590

Edited by ZX81 on Sunday 20th March 07:04

Edited by ZX81 on Sunday 20th March 07:06

[Belle427]

You need to set the base idle with this, clamp the big rubber hose to the right of it and set the idle to around 600 rpm, I can’t remember the exact figure sorry.
Can be tricky to keep the engine running if it’s been set too low with the hose clamped so I’d wind it out a turn or two first.