P38A GEMS idle problem

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Interestingly enough, the original issue I had (and have had for months) is actually still there after all this work.

That is, once the engine has run long enough to warm up (not even to operating temp, so still just in the blue zone) if you switch off (for instance to pump fuel), when you restart it fires up immediately, idles fine then after a few seconds the revs drop really low and it stalls.
If you rev it to 1K or so for 10 seconds then all is ok, it drops back and idles. Never happens on a cold start, only on 95% of warm starts.

Driving after that there are no problems.

No errors at all on ECU.

Any thoughts?
 
Hi Guys
Bringing this thread back to life sadly as I have issues again. Sorry if it appears twice with slightly different wording, my session timed out and I needed to log in again (losing what I had written).

Started with routine maintenance and cleaning in the engine bay.
Basically the MAF cable was broken, cracked open circuit halfway along it. It was taped up by somebody a long time ago, must have been before I had it, so maybe 10 years. It may account for the occasional drop in idling revs I have noticed on the odd occasion. I stripped the cables on either side of the break and resoldered them.
Scanned for ECU errors and got a MAF out of range error, cleared that.

To start it I have to tickle the accelerator and hold the revs at 1500 or so for 10 secs it then drops back to normal idle and stays there ok if I do not do this is stalls.
But when driving it now wants to cut out at most junctions as opposed it idle.

Cleaned MAF
Cleaned IACV
Cleaned throttle throat.
Clean all sensor plugs and sockets.
2 new breather pipes from the rockers to plenum.

Scanned with GEMS Lite and it says the TPS closed position is 0.64, it should be 0.85.
Reset adaptive values and it confirms TPS close position now set to 0.85.
Now idling perfectly at 700 rpm.
Switch off, no change on start needs tickling on the accelerator.
Read with GEMS Lite again and problem back as before. Reset and all ok idling until I switch it off.

Unplug MAF while running and revs drop off, back in and it climbs back to normal idle.
Unplug IACV while running nothing happens.
Unplug TPS when running the revs climb, plug back in it drops back to idle.

Any ideas about what may be doing this?
Thanks in advance
James
 
I have two GEMS engines one TPS reads 0.58V at idle and the other 0.62 the "current" value should equal the "stored" value else it will cause problems. The 0.85V sounds quite high tbh. You should be able to pick up the TPS voltage even with engine off - to demonstrate that it does what it should.
Worth sharing the other data points for a warm idle eg IACV steps (22+/-7), MAF/ Air flow sensor (volts, about 1.4/1.5V at idle) and O2 sensors. (incl fuel trims) Are your MAF, IACV and TPS all genuine items? Before the 02 sensors come on line the fuel trims shd go to 38.75% - I wonder if yours is doing that - are the temp sensor readings out? Does the throttle butterfly physically close, or is it getting stuck (0.85V indicates it may be...). Just a few ideas, easier if you have diagnostic access
 
romanrob said:
I have two GEMS engines one TPS reads 0.58V at idle and the other 0.62 the "current" value should equal the "stored" value else it will cause problems. The 0.85V sounds quite high tbh. You should be able to pick up the TPS voltage even with engine off - to demonstrate that it does what it should.
Worth sharing the other data points for a warm idle eg IACV steps (22+/-7), MAF/ Air flow sensor (volts, about 1.4/1.5V at idle) and O2 sensors. (incl fuel trims) Are your MAF, IACV and TPS all genuine items? Before the 02 sensors come on line the fuel trims shd go to 38.75% - I wonder if yours is doing that - are the temp sensor readings out? Does the throttle butterfly physically close, or is it getting stuck (0.85V indicates it may be...). Just a few ideas, easier if you have diagnostic access
Click to expand...

Thanks.
The 0.85v I quote comes from page 16 of the GEMS by Poole manual as supplied by Saint.V8. It also states lower values may result in poor idle characteristics. That is all I have to go on.
I will double check the TPS voltage with the engine off again.
I am not sure I have anything that will read the IACV steps.
All components are genuine OEM items.
I need to check the temp sensor readings. Do you mean engine temp or the sensor on the filter box?
The butterfly does seem to fully close and springs are ok.

I have been thinking of getting some more effective diags, without spending too much what is recommended as both effective and good VFM? Lynx? Nanocom?

Cheers
 
ours was acting up with similar symptoms, not wanting to idle/needing throttle to keep it going, put a new crank sensor in as the heat shields are missing on that side so was thinking its got hot and maybe gone wrong and wasn't a lot of money and easy to fit, no different, so tried a new coil pack and that sorted it.
 
JamesBB said:
Thanks.
The 0.85v I quote comes from page 16 of the GEMS by Poole manual as supplied by Saint.V8. It also states lower values may result in poor idle characteristics. That is all I have to go on.
I will double check the TPS voltage with the engine off again.
I am not sure I have anything that will read the IACV steps.
All components are genuine OEM items.
I need to check the temp sensor readings. Do you mean engine temp or the sensor on the filter box?
The butterfly does seem to fully close and springs are ok.

I have been thinking of getting some more effective diags, without spending too much what is recommended as both effective and good VFM? Lynx? Nanocom?

Cheers
Click to expand...
It aint cheap, but the Nanocom is the best and can always be resold for a goodly sum as it's not VIN locked.
 
I do not think its coil pack related, it was all ok until the MAF cable fell apart and I resoldered it.
Checked TPS voltage, back to 0.65v before attempting to start. Looking at other reports and some Youtube vids it seems to not be unusual to see the TPS down below half a volt at closed. Maybe I need to check my 5v reference to see that is actually 5v....
Starting symptoms are like it's not reading the MAF properly. I have tried starting with a disconnected MAF before and its not easy and needs tickling the gas again. Reading MAF via Torque shows some air movement being measured, no idea how much. Once its idling ok all looks great until I go to move it then it stutters and cuts out.
Using Torque to check the response on the TPS and that is linear when graphed.
I am wondering if the broken cable actually has another bad spot on it that could have gone high res....... Before re-wiring that area of the loom I am thinking of seeing if I can borrow another GEMS MAF to eliminate that first.
Things on my list.
  • Check 5v reference.
  • Borrow, acquire, buy a 2nd hand MAF to test/eliminate.
  • Check cable along the whole length for signs of damage.
  • Buy better diags. Looking at reviews on various forums and on videos I see that Lynx is hard to beat, it has worked more reliably than some of the Black Box Solutions diags, and its cheaper and has very good reviews generally. Birthday coming up, speak nicely to Missus.
  • Hope for some sun or at least dry weather.
Cheers
 
James - GEMS by Poole is a Morgan doc, at the end of the day. I helped an rr.net member out today - her TPS was also 0.6V. Torque I don't know, but on proper diagnostics you would have a target voltage and a current voltage at idle - the two have to be the same. I think you are measuring the voltage on a DVM (?), so can't see what the ECM has as the target voltage for "throttle shut". The temp to measure is the Engine Coolant Temp - not the IAT one (one the air filter).

Re diagnostics - Nanocom all the way - to be fair I've not had any other, but on the other sites I inhabit Nano is now the perceived wisdom for a P38, although admittedly there are still a couple of tiny things BBS can't be bothered to fix.
 
romanrob said:
James - GEMS by Poole is a Morgan doc, at the end of the day. I helped an rr.net member out today - her TPS was also 0.6V. Torque I don't know, but on proper diagnostics you would have a target voltage and a current voltage at idle - the two have to be the same. I think you are measuring the voltage on a DVM (?), so can't see what the ECM has as the target voltage for "throttle shut". The temp to measure is the Engine Coolant Temp - not the IAT one (one the air filter).

Re diagnostics - Nanocom all the way - to be fair I've not had any other, but on the other sites I inhabit Nano is now the perceived wisdom for a P38, although admittedly there are still a couple of tiny things BBS can't be bothered to fix.
Click to expand...

Agreed on the GEMs by Poole, but it was recommended by a reputable source and is essentially the same engine management.
Reading voltage with GEMS Lite not a DVM.
Engine temp was measuring about 45-50 degrees at the time I last checked so warming up.

Understand Nanocom Evo is new standard, however, its a crazy price compared to an entire P38. I will probably only use it once a year so maybe the cheaper diags will do enough. Need to think on that, it's a lot to invest for annual usage (resell value considered or not).
 
JamesBB said:
Agreed on the GEMs by Poole, but it was recommended by a reputable source and is essentially the same engine management.
Reading voltage with GEMS Lite not a DVM.
Engine temp was measuring about 45-50 degrees at the time I last checked so warming up.

Understand Nanocom Evo is new standard, however, its a crazy price compared to an entire P38. I will probably only use it once a year so maybe the cheaper diags will do enough. Need to think on that, it's a lot to invest for annual usage (resell value considered or not).
Click to expand...

James, once you have it, you'll use the nano a lot more than you think, and it saves a lot of diagnosis time when things go wrong. So it's just a trade-off of time vs money really. It fits easily into the glove compartment too. That said I have two gems units, so can use it on both

The GEMS by Poole doc is an excellent doc. I think if you read carefully (I had to read it a few times too) it is saying that after you reset adaptive settings it will default to 0.85V before re-learning the actual value. Nanocom also has an option to set a target tps voltage (which the ECM will over-ride in any case). BTW, even if you don't have a nanocom you can download the system documentation, and there are some useful tit-bits of info in there too...P38-specific. If you can't get at it, drop me a msg with your email and I'll send it over
 
Last edited:
Some readings after 5 mins of good idling:
Coolant temp = 61C
Intake temp 22C
RPM = 625
TPS Volts = 0.65v
Short term fuel trims bank 1 = 24.2
Short term fuel trims bank 2 = 24.2
MAF (CFM) ranges from 0.5 to 1.2 which seems amazingly low.
If I unplug the MAF nothing happens at all now.
Plug it back in there is a blip of revs (nothing much at all). I am guessing that is wrong.......

Ooooo oooo, more info.
Get it idling smoothly and then unplug the MAF and its fine.
Put it into reverse and drive and a quick spin around the garden and all is ok. No fatal drops in revs and no stalls.
I hope that helps prove (to me at least) that I need to try a 2nd hand MAF ERR5595A, its about 15 years old after all..... Report back once at that stage.
 
yeh - MAF shd be around 1.5v at idle +/- 0.1v. 61 degrees seems low for coolant temp - is that cos it still wasn't warm - otherwise that's an issue and the sensor could need replacing. RE sensors. I guess you prob have zero ltft for now, if you have recently reset adaptive values.
 
Have you done a compression test?

I had (and still have) rough running when warm and sometimes problem starting and it's turned out to be low compression on 7 of the 8.

I had a duff TPS that needed replacing, and I did the CPS too for good measure. I had a leaking exhaust manifold gasket that I replaced, new plugs and other bits and bobs. Everytime it made a little difference, but never quite cured anything. In the end a compression test showed I have major engine wear and that was probably the biggest problem all along!!
 
Hi All here is an update.
I swapped the MAF for one from a scrap yard and its all back to normal.
Starts instantly and idles around 700 pm, in fact, it has never idled so smoothly.

Picked up loads of spares too from the old P38 that was being torn apart.
TPS, IACV, all temp sensors, grabbed a few spare relays (inc cruise control relay) loads of trims (internal and external), clips, RF receiver, dash trims, binnacle surround, indicator stalks, door locking module from driver's door, loads of other bits and bobs, all for about £50.
Bargain.
 
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