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Marlin Sportster, Cabrio, Berlinetta and Roadster builds Enthused or Confused about your vintage Marlin build? Ask away here or show off your build.

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Old 21st December 2015, 20:40
NigelB NigelB is offline
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Default I vote for a hunting ban.

Nothing to do with horses and hounds...........

But a problem I have had ever since I built the car and has just re-appeared recently.

Some of you may remember me talking about my engine hunting at idle. Once I had applied some throttle the engine revs were stable but as soon as I returned to idle, it started to hunt again. And this was no random idle it was consistent between 800 rpm and 3000 rpm at about 20 cycles per minute. I would start the car, stop it, start it again and stop it with no change and then on the next start, with no apparent reason the hunting would stop and everything would be fine. I've checked all around for vacuum leakage and renewed any dodgy rubber hoses etc and I'm fairly confident that it is not that.

I am however becoming more and more confident that it is linked to disconnecting the battery and re-connecting it after a period of a few hours or more. I think it happens less when the battery is only disconnected for a few moments.

I've heard talk before of and ECU 'learning' about the engine settings. Could that be significant and that learning is lost when power is removed. But why would anyone build an ECU with a volatile memory. Is the ECU perhaps losing a reference value from somewhere when it is powered down that has to be re-established over a period of time. And I've recently remembered (Mike) that we hard wired the switched earth to the fuel pump relay and the oxygen sensor when we thought it was wrong that the fuel pumps weren't running when the ignition was on. I think you have reverted to the correct wiring but I have left mine hardwired. So am I getting some earth leakage which is erasing any of the learning or reference values I should have.

All speculation at the moment and I need to restore the wiring to provide a switched earth, but does anyone else have any thought of what may be causing the hunting once the battery is reconnected.

Maybe I don't want to ban it, just fix it.

Many thanks

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Old 21st December 2015, 21:36
phil.coyle phil.coyle is offline
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Hi Nigel,
The ECU "learning" is a posting I put on the Marlin Owners website.
Basically you just turn the ignition key to the ignition only, engine off, and then press the accelerator pedal fully down and up quite quickly.
Depending on the age of manufacture of your particular vehicle it may take 10 presses up and down and sometimes less, a bit of trial and error.
Switch the ignition off and finish your cup of tea (about 2 minutes).
Now start the engine, all this does is give the ECU the values of the throttle position sensor, full throttle and tickover.
The ECU is already programmed to the values of the TPS and all you are doing is refreshing it's memory, it probably gets bored with the hundreds of thousands of times it has been pressed up and down or held in one position for ages.
Another trick is to disconnect both battery leads and then somehow connect the earth lead only to a suitable earth in your garage or metal drain cover ( connect jump leads or long piece of wire to water pipe) after your second cup of tea reconnect battery, reset TPS, and keep your fingers crossed.
If you think I'm a looney that's fine you will not be the first, but I know from experience what works.
You cannot have the fuel pump running continuously when you switch on. The ECU runs the pump for 3-4 seconds and only when you crank the engine does the pump run continuously.
Think of it this way, if one of your injectors was faulty and had stuck open, when you switched on you would have fuel at 45psi or 3bars going straight into the cylinder, it would not take long to fill it up. That will give you something to think about.

Last edited by phil.coyle; 21st December 2015 at 21:54.. Reason: Had to nip out
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Old 21st December 2015, 22:03
NigelB NigelB is offline
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Hi Phil,

There's no such thing as a lunatic idea when

a) your knowledge is as limited as mine and
b) you've already tried loads of stuff and it hasn't worked.

I like your idea. Its got to be worth a try. It definitely seems as though there is a reference being lost when the battery is disconected and something needs to be done to restore it. I'll give it a go when I'm brave enough to disconnect again.



And yes Phil.I now know that the earth for the fuel pump should only be available once the cam position or crank position sensor picks up the rotation of the engine. Learning all the time!!

Last edited by NigelB; 21st December 2015 at 22:08..
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Old 22nd December 2015, 09:26
phil.coyle phil.coyle is offline
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Hi Nigel,
If you want something to think about whilst chewing on your turkey leg, try this.
Ignition on.
With your multimeter test the 3 wires coming from/going to your TPS.
Your favourite destruction manual should give you the colours etc..
One is a feed from ECU either 5v or 9v depending on model, one is the signal wire return to the ECU, and that leaves the earth return.
At rest (closed throttle) the signal wire should show about 0.5v.
At full throttle 4.5v, anything higher at full throttle 5v.
or anything at closed throttle like 0.1v means a faulty TPS. The 9v version is relative.
It's about 5% to 95% of your operating voltage for your signal return wire.

The ECU cannot accept 0v. or 5v/9v as in digital speak it means on or off and the ECU is programmed to accept voltages from 0.5v to 4.5v..
Don't panic!
p.s. don't get too hung up on disconnecting your battery, the programme in the ECU is permanent. Any faults that develop will remain in the ECU no matter how many times it is disconnected, only by using a code reader can you remove or alter any info stored in the ECU, but you can change little things like TPS resetting.

Last edited by phil.coyle; 22nd December 2015 at 09:43..
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Old 22nd December 2015, 11:53
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peterux peterux is offline
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Sorry to hear that your still having hunting issues.
Do you still have the BMW diagnostic connector in your engine loom? If so you can get a code reader from a company called Peake.
They do come up on ebay from time to time but check the Peake website to make sure you get the right version. You can read codes and reset them to fault find. Very useful!

I had a hunting problem on my M52B28 (MS41 DME) but only in the 400 to 600rpm range and a cycle time of 2-3 seconds. It only happened after I changed the DME to a EWS deleted version. It turned out to be the engine temperature sender. I'd try checking the temp sender wiring connector first as it may be intermiitant and causing the DME to think the engine is cold. Incidentally, I had it confirmed that the MS41 DME adaptations are not lost when de-powered.

Good luck, Peter
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Old 22nd December 2015, 12:22
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MartinClan MartinClan is offline
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The fact that it went away and then came back must have some bearing on the problem. I suspect a sensor issue and/or associated wiring. I would double check everything and in particular earthing. Disconnecting the battery may be a red herring - I had plenty of those when trying to diagnose my misfire last year.

Some sensors are "optional" with the ECU reverting to default values. If you know/can find out which sensors are optional I would start by disconnecting them.

Cheers, Robin
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Old 31st December 2015, 10:49
mcramsay mcramsay is offline
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When I got my m3 lump running the idle was not stable at all, I spent ages looking for vaccum leaks, and found none, finally I hooked up the BMW diagnostic gear, it pointed me in the direction of one the lambda sensors. On further analysis the signal cable on one of the sensors had rubbed through, this was giving sporadic readings when the engine was vibrating, this then caused the idle to hunt as it was receiving duff fuel info on one bank. May be worth checking? The cabling is easily fanged when installing the engine
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