Running Issues M20 post facelift

[HairyScreech]

First off, yeah I know, should be able to solve this by myself but i'm a bit stumped and short on time.

Got a bit of a running issue I can't get to the bottom of.

About 2 months ago the car started missfiring under load at around 5k, it was intermittent but when it happened it was like hitting the rev limiter.
Changed the fuel pump and the problem seemed to clear up.

Recently I have been getting a slight hesitation, inconsistent but certainly present, doesn't seem to be at any one point in the range or temperature related.
I thought this might be fuel related again and had a sniff around the filter, one 1989 vintage filter replaced and out for a run to work this morning.

Seems I still have a hesitation and after sitting at a set of lights changing to 2nd resulted in such a back fire I actually crawled under the car when I got to work expecting to see some serious damage.

I checked the basics when I started looking for the misfire, blue sensor I believe is ok (6 months no more) and I have checked the plug under the manifold, the TPS gives the idle position click, crank sensor is fine, no difference in the starting behaviour.
I put a new Bosch coil on it as they were cheap from ECP so its not the coil.

Pulled plug 2 and 4 yesterday and both look clean as a whistle, put a boroscope in there and there's a healthy mm or so of carbon on the piston tops so I'm ruling out a real solid problem like a failed blue sensor or dead maf.

A bit of research says that there should be no lambda relay in the DME box as there is no sensor on the car, I believe one has been there from day 1, (had this one quite a while).


Any ideas what could be causing this?
I'm going to check the ICV and MAF later on.

[Grrrmachine]

The trouble is that you're swapping bits instead of testing, and that makes it very difficult to diagnose. FOr starters, have you tested the blue temp sensor at the ECU? Have you taken the plug off the TPS and confirmed the change in resistance when it switches? Have you done a resistance check on each of the HT leads, and systematically checked for spark at each plug? All of these can be done in an hour, for free, before you start swapping in new bits.

From there, pull your injectors and send them to injectortune. And I'm fairly sure sparks shouldn't be clean as a whistle, nor should there be 1mm of carbon in the piston.

[HairyScreech]

Ok, To clarify, "sure sparks shouldn't be clean as a whistle, nor should there be 1mm of carbon in the piston." - Plugs a light dusty brown, carbon build up perfectly normal for a road engine, carbon will build on piston crown to around that due to the quenching effect of the pistons cooler mass on the end gasses. From inspecting the chamber its exactly as I would expect for a correctly running motor.
What I conclude from it is the plugs are not fouling, there is no fuel washing of the chambers and its not consistently lean.

Blue sensor wiring to the ECU was fine when I checked not long ago, would be surprised if its gone in the last few months.

Tested the TPS when the issues started, at work and can't remember if it was resistance when open and none at idle or visa versa but again it would have had to have failed since.

I have not put a meter down the leads yet, this is an across all cylinders issue rather than a single cylinder thing.

With regard to swapping parts, the fuel pump was noisy and the filter was a 230k part even if it wasn't them then they were contributing, cut the filter open and it was black.
The coil was a just while I was there job, another 1989 part, only mention it to rule it out.

What ever it is then its effecting all cylinders intermittently, its a there one moment and gone the next fault.

[Grrrmachine]

Yeah, that's a better desciption, cheers

From all that I'd expect your AFM might be going duff, as in a worn carbon track, unless you do have a MAF like you wrote in your last post. I doubt the lambda relay is doing any harm.

[daimlerman]

I'm getting a similar issue with my M50!

An odd misfire that occurs randomly,today is was whilst travelling through roadworks at low speed...2nd gear,1500/2000rpm.Outside temp was enough to fry an egg on the bonnet.

In the past,it has occoured when at full throttle and overtaking...

At all times,lifting off and re-applying throttle clears it.

I am almost convinced that I have an intermittent ECU fault,I'm waiting for a s/h one to arrive to try.

Given the similararity between M20/M50 ECU,could you be suffering a similar problem?
I have no recolection of M20 ECU issues,but there are now several M50 ECU issues mentioned.

Just a thought,Byron!

[HairyScreech]

Grr - Touche yes AFM rather than MAF.

Malc - Odd, sounds exactly like I have. I am pretty convinced its electrical or fuel related rather than mechanical (ie, dead ht lead, stuck injector, cracked dizzy cap).
[edit] - [perhaps there is some clue here, sounds like you have the same issues, so things that have changed on the m50 can be ignored at first guess. which is mostley the ignition side of things, something I was fairly sure it wasn't, lets see if the theory holds up]

When I think about it the lambda relay came with the loom (fried the loom doing something like a tit.) the loom came from a post 91 car.
I have heard suggestions that with a relay in the ecu thinks there should be a sensor.

Certainly I have no missing on full throttle/open loop. I have even been out trying to get it to play up.

Going to try resetting the idle by disconnecting the battery as my main concern was the huge exhaust mounted explosion after being sat at a red light, seems like it filled the exhaust and lit it off when free air got in on the gear change.
Wondering if this is simply a symptom of the ecu fiddling the idle as the pressure fell and now its got full pressure the idle settings its learnt are too rich.

[Brianmoooore]

Odd, sounds exactly like I have. I am pretty convinced its electrical or fuel related rather than mechanical (ie, dead ht lead, stuck injector, cracked dizzy cap).
[edit] - [perhaps there is some clue here,

There is a clue. If it had a massive backfire, then there's fuel/air mix getting into the exhaust system, without being ignited by a spark. Distributor cap/rotor arm would have been my starting point.

Lambda relay fitted won't do any harm - ECU doesn't know if it's there or no, and it's just switching power to nowhere if no sensor is fitted.

[HairyScreech]

Ok, been out and had a fiddle, I can tell you what it isnt.

Its not the ICV, cleaned and tested.
Its not an air leak
Its not the TPS - gives and breaks continuity just fine
Its not the dizzy cap/rotor, bit furry gave it a clean and adjusted the pins to close the gaps a touch.
Its not the lambda relay, pulled it out just in case.
I don't think its the AFM, wiped the carbon tracks off as they were a bit dirty, resistance is 550 ohms on the IAT sensor (about 30 deg in the bay when I first checked) the flap gives a reasonable smooth resistance change from 1430 to 2400 ohms, this a bit low? I do have another somewhere.
Leads seem ok. Around 1910 ohms.
As said before coils new.


Just taken it for a drive and its exactly the same, need to recheck the blue sensor and the CPS.

[AardHawk]

I dont trust universal replacement coils. What are your coil resistance values? Primary terminals 1 & 15, and secondary 15 & 4 (centre).
6 cylinder engines, Jetronic 0.85 & 8250, M1.0 0.5 & 6000 and M1.3 0.5 & 5000.

[HairyScreech]

Its a genuine Bosch e30 coil. Only difference from the dealer part is the box it comes in.

I will check that though on both the old coil and the new just for the info.

It seems a touch better this morning in the colder air.

Need to recheck my blue sensor values, got a feeling I might regret fitting a Lucas part (all I could get at short notice last time it went no more than a year ago).
If it is out then its gone since last time I checked and at a total coincidence to the pump failure.