Intermittent misfire/stall

[MarkKehoe 1]

Hi all, I have an intermittent misfire that's driving me crazy. The frustrating thing is that the car is running like a dream, it ticks over at a rock steady 1000rpm and pulls like a train, then completely randomly it will just die for a second and spring back to life.  Occasionally after this happens it will die all together and struggle to start but then it's fine again. When it does start it starts crisply with no bogging or indication of unburnt fuel. So far I've changed plugs, leads, new distributor, new coil, new temp sender, new tps, new tachometric relay, new fuel pump, low miles on afm, ignition timing set, new timing belt, checked earths. 

Any thoughts on what it could be? Ecu? Immobiliser? Should I be running resistor plugs? Any help would be much appreciated, I'd love to get it sorted for a few events over the summer.

 

[PhilNW 40]

Start by establishing which cylinder(s) is/are misfiring and whether it is fuel or electrical related.

 

Pull the plugs leads off 1 by 1 (be careful of shocks) and see if the misfire gets worse or not, and whether there is a spark or not.

 

Also take the injector connectors off 1 by1 to see if it injector related.

 

"it will just die for a second and spring back to life" suggests it's electrical or earth,

 

I had a problem once which was the earths on the injectors under the manifold. so wriggle the wiring to see if it is a bad wire/connection. 

Edited June 3, 2023 by PhilNW

[DamirGTI 323]

Possibly ignition module overheating , if not failed completely already .. try to apply some thermal transfer paste at the back of the module and see what happens .

 

If not i'd be tempted to try an spare known good ignition module .

 

Also , could be dizzy signal cable .

 

D

[MarkKehoe 1]

Thanks Phil/Damir, changed the ignition module but it wasn't oem so I sourced an oem but still didn't resolve the issue. Both were fitted with thermal paste. The new distributor was sourced from h&h but they no longer have signal cables so I reused the original which was in good nick and checked with a meter. I've wiggled the loom at injectors and afm but can't replicate the issue.

[PhilNW 40]

So it is not misfiring at the moment?

 

Have you taken it for a run to get it really warmed up?

 

 

[MarkKehoe 1]

Hi Phil, took it for a 15 mile spin last night, so well warmed up. It only died/missed twice in the whole drive so literally 2-3 seconds out of a 20 min drive. It's not like it missing on a cylinder, the whole engine dies but then comes back to life in a sec or two. No evidence of the tacho bouncing when it dies? 

[PhilNW 40]

Try to establish under what circumstances the engine stops (and goes again). under acceleration? on throttle, off throttle, speed etc

 

And what, if anything, you did  do or happened during that few seconds?

[MarkKehoe 1]

Last night both were mild acceleration out of bend/roundabout, so about 3-4k rpm. Earlier in the week I brought it for a similar drive and it never missed a beat. I took it home washed it and went to bring my son for a spin later that evening and I didn't get half a mile down the road before it started misbehaving (same misfire), on that occasion I had to pull in and struggled to get it to start. The problem is it's totally random, I haven't changed any parts since that drive and the spin last night? 

[PhilNW 40]

So it is more of a loss of power to the point where the engine dies rather than a single cylinder misfire.

 

Did you do anything different when it struggled to restart other than repeated attempts with the starter?

 

Next time it stops see if there is any spark  and check the injectors are operating by putting the tip of a long screwdriver to each injector, put your ear to the handle and listen for ticking sounds when turning over.

 

Sounds like something electrical or overheating and then cooling down when you stop, question is what?

 

I had a similar problem which ultimately turned out to be a sickly fuel pump delivering low fuel pressure especially when hot. However you have already replaced the pump so we  can dismiss that.

[MarkKehoe 1]

Thanks Phil, I guess I'll just have to wait until I get an opportunity to check injectors and spark. I managed to catch this video clip (which was very mild) not long after taking off this evening. 

[PhilNW 40]

My first reaction to the video is it is fuel related  I know you said you have replaced the pump , what make of pump is it ?

 

Can you check the fuel pressure? should around 3 bar..

[MarkKehoe 1]

New bosch fuel pump and new filter. I've never checked the fuel pressure to check if the regulator is OK. I'll run a pressure gauge in line to see what I get.

[Tom Fenton 1,524]

Sounds like air flow meter to me. Engine still running but not responsive.

[MarkKehoe 1]

Thanks Tom, I've been reluctant to break the seal on the top of the airflow meter. It's a refurbished unit that was fitted to the car probably no more than 4000 miles ago but it was in dry storage for about 10 yrs. The CO2 is set to 1.5%. Do you think it's worth breaking of the top cover to check track and connections? 

[welshpug 1,621]

you don't need to break the seal to test it.

[PhilNW 40]

If you look at the AFM have a look at the terminals on the wiring, they caused me a load of grief at one time until I changed them all.

 

Remind yourself (cos I do) that the wiring is about 30 years old and go from there!!  

[MarkKehoe 1]

Quick update, I checked the pressure regulator which was almost exactly 3 bar. Then I checked the afm, I got 189.4ohms between 8&9 and 364.7ohms between 5&8 which is within spec. Terminals 5&7 measured 173.9ohms to 1.329k ohms which seemed high so I took off the top cover cleaned the track and metered all of the connections. The resistance dropped into the range 118.4 to 965 ohms which is in spec. I took the car for a spin and it ran very well. The bad news is this evening it's playing up again,  I rechecked terminals 5&7 and there seems to be a consistent open circuit at one point on the track. It's difficult to say if this is meter lag but it does seem to be consistent with that spot. The resistance had also gone up again but in hindsight temperature is probably a big factor. 

[MarkKehoe 1]

[PhilNW 40]

Is the fuel pressure good over the whole rev range?

 

Not sure if the open circuit explains the situation were "it will die all together and struggle to start but then it's fine again", unless the flap is sticking at that point.

 

Can you hold the engine revs at a level were the track is on the open circuit position to see what happens? Should be repeatable

[Tom Fenton 1,524]

I think I'd try another air flow meter at this point if you can get hold of a known good used one.

[ScoobyJawa 22]

I had a very similar issue in mine, changed ignition amp and it got better, then changed the coil and touch wood it’s been fine ever since. I know you said you changed it but maybe it’s duff? I had exactly your symptoms, misfire and cutting out, then restarting fine….

[PhilNW 40]

1 hour ago, ScoobyJawa said:

I had a very similar issue in mine, changed ignition amp and it got better, then changed the coil and touch wood it’s been fine ever since. I know you said you changed it but maybe it’s duff? I had exactly your symptoms, misfire and cutting out, then restarting fine….

Had the same thing on mine but it was the 30 year old fuel pump at the end of the day.

Edited June 7, 2023 by PhilNW

[ScoobyJawa 22]

To be fair it had a new fuel pump too, but that completely failed and the car wouldn't even start!

[MarkKehoe 1]

Update on fuel pressure, 3 bar with ignition on. 2.5 bar at tick over and fluctuates between 2.2 and 2.8 bar as throttle applied and manifold pressure varies. When vacuum from manifold disconnected from regulator it sits at 3 bar.

[MarkKehoe 1]

I also rechecked the afm with the ignition on and measured voltage between 5&7, the result was a smooth 1.5v to 7.5v at flap fully open. I put the meter in manual because I think the auto ranging was causing issues.