M20 stalling at idle and bogging down

Post Sun Sep 14, 2014 10:45 pm

Having a few more issues, the m20b25 will start and idle fairly smooth but if you rev the car it will do one of two things, either the revs drop and it stalls or it almost stalls but then recovers but it never returns to a nice idle.

If I pull off in the car it just bogs down and splutters , it will drive through it but then it will just big down again higher In the rev range.

Is this a fuelling problem?

Cheers

Post Mon Sep 15, 2014 2:17 pm

or CPS maybe?

Post Mon Sep 15, 2014 2:44 pm

Try disconnecting the ECU or battery and leaving it for an hour or so to reset it. Reconnect, let the engine idle for at least 10 minutes to enable base idle settings to be learnt. Switch off, then go for a drive (if possible) throughout the rev range.

Post Mon Sep 15, 2014 2:48 pm

That's interesting, the battery had been off and on while the alternator fault was sorted.

I'll disconnect it again and start from scratch.

Would that have anything to do with the splutter under load?

Post Mon Sep 15, 2014 3:02 pm

Quite possibly, if the base idle settings haven't been learnt. Also check that the TPS is working:

http://www.e30zone.net/e30zonewiki/inde ... ake#Idling

Post Mon Sep 15, 2014 4:20 pm

Randomly the battery light on the dash has stopped working when I just or home to test letting it idle for a while

Post Mon Sep 15, 2014 5:17 pm

Use a multimeter to check the voltage across the battery; with the engine idling, the voltage should be around 14V. If it's lower, try revving the engine slightly and see what the voltage is.

Are you sure the alternator fault is fixed?

Post Mon Sep 15, 2014 6:11 pm

Swapped to a different alternator, got around 13.5v volts and the battery light is working again but the idle bogging down fault hasn't changed between either alternator.

I have recorded a short clip which I will upload to YouTube

Post Mon Sep 15, 2014 6:19 pm

Have you tried a different (known good) battery?

Post Mon Sep 15, 2014 6:20 pm

I put a brand new battery on it last week in case they was an issue

Post Mon Sep 15, 2014 6:28 pm

Strange. It should read at least 14 volts or so when charging.

Check the earths of the battery, alternator and engine-to-body earth strap.

Post Mon Sep 15, 2014 6:31 pm

I have cleaned all the terminals on the battery, the leads to the battery , between the battery and the strut and re crimped the two connections to the alternator

I'll take off the strap between the inner wing and the engine and clean those up, is there any where else the check?

Post Mon Sep 15, 2014 10:03 pm

I think many E30s had an earth strap between the engine block and a chassis leg, but maybe yours just has the one on the inner wing...

Is the alternator belt tension correct? There should be about 1/4" of play when pressing in the middle of the longest run between pulleys.

You say you fitted a different alternator, but is it a known good one? May be worth pulling off the brush pack and checking their condition.

Post Mon Sep 15, 2014 10:29 pm

I had a quick look, it is to the chassis leg I just hadn't paid much attention to it.

There's roughly 1/4inch of play, maybe a little less so I could always slacken it off a little bit.

This alternator was fine before the engine was swapped so I have put it back on as I knew it to be ok 6 months a go, I replaced the voltage reg before putting it back on.

Post Mon Sep 15, 2014 10:57 pm

What happens if you put the headlights on with the car idling, then rev it a little - do they glow noticeably brighter?

Post Mon Sep 15, 2014 11:28 pm

I'll try that tommorow

I know a belt can be too loose but can being too tight have the same effect? The belt at present can be twisted 90 degrees almost so it's can't be that tight I don't think

Post Tue Sep 16, 2014 10:48 am

It will just put added strain on the pulley bearing(s) if it's too tight, and wear out the belt quicker, but it won't affect the charging.

Post Thu Sep 18, 2014 8:32 am

Here's a link to how it runs

Had a new set of plugs last night

Checking cps tonight

Post Thu Sep 18, 2014 6:45 pm

Checked cps

Showing 860 ohms, according the wiki it should be 540 +/- 10 so can I assume this is at fault?

Post Thu Sep 18, 2014 9:54 pm

Possibly. Where did you check it, at the CPS or the ECU?

Post Thu Sep 18, 2014 9:57 pm

On the CPS lead

Post Thu Sep 18, 2014 10:00 pm

Interesting. It's only a coil, so they normally work fine or don't at all. Could be worth measuring it at the ECU socket just to confirm that...

Post Thu Sep 18, 2014 10:05 pm

I'll have a look tommorow but I ended up ordering on a next day with it being so far out of tolerance.

I'll report back tommorow evening and see what happens

Post Fri Sep 19, 2014 6:57 pm

I changed the cps this evening and it now idles and revs nicely

I drove it and it was fine until I turned the lights on then it immediately spluttered and tried to die until I turned them off , the remained off and then it tried to die again and wouldn't recover until the revs really dropped

It's like it's running out of fuel, the idle is solved but the platter isn't

Post Fri Sep 19, 2014 9:54 pm

Could be worth disconnecting the battery or ECU and leaving it for an hour or so to reset it. Reconnect, let it idle for at least 10 minutes to enable base idle settings to be learnt. Switch off, then take it for a drive throughout the rev range.

This enables full data transfer from EPROM to the processor.

Also check that the battery is being properly charged; it should measure 14V across the battery with the engine idling.

Post Fri Sep 19, 2014 10:09 pm

Tried the disconnecting battery but it doesn't help.

It's just like it runs out of fuel, only get 13.5v across the battery on idle

Weird problem

Post Fri Sep 19, 2014 10:20 pm

Speedtouch wrote:

What happens if you put the headlights on with the car idling, then rev it a little - do they glow noticeably brighter?

Could be worth changing the fuel filter.

Post Fri Sep 19, 2014 10:54 pm

No, they stay the same

It idles fine now, just dies under driving load really

I'll get a fuel filter on it in the morning, it's under the drivers side near the wheel arch isn't it?

Post Fri Sep 19, 2014 10:54 pm

Speedtouch wrote: Also check that the TPS is working:

Post Fri Sep 19, 2014 10:55 pm

Is that a physical check or ohms?

Post Fri Sep 19, 2014 11:04 pm

Both. They tend to fill with oil.

Post Fri Sep 19, 2014 11:07 pm

I'll whip it off tommorow, do you need a new gasket before refitting?

Post Fri Sep 19, 2014 11:10 pm

For the whole throttle body? In theory, yes. In practice, no, as long as it goes back exactly as it came off.

Post Fri Sep 19, 2014 11:12 pm

Thought so, same as some of the gaskets in my job .... Just a carefully does it removal

Post Fri Sep 19, 2014 11:14 pm

Mne666 wrote: I'll get a fuel filter on it in the morning, it's under the drivers side near the wheel arch isn't it?

Passenger side rear, on the M20 E30s I owned. Depressurise the injection system before attempting to remove it, by removing the fuel pump relay and running it until the engine cuts out. And be prepared for a squirt of fuel out of it - best to wear gloves and goggles.