Strange misfire (PI car)

[Red6]

Did a 134 mile round trip this morning, the majority of which was motorway. First 67 mile leg was fine and the car cruised happily at 70 on the motorway in overdrive. The car sat for a couple of hours and when I came to drive home it had developed a misfire. Car would tick over with a slightly lumpy idle but nothing too drastic. When I tried to pull away there was a definite misfire up to around 2,500 rpm at which point it seemed to start running on all cylinders again. On the motorway it seemed fine unless I let the revs drop but on the A and B roads, at 2,000 rpm, the misfire was a steady pulsing, running OK for 3 seconds then misfiring for 3 seconds, running OK for 3 seconds and so on. When I got home I checked the injector pipes and they all had a reasonable pulse. Let the car cool down for an hour and then took the plugs out and they all seemed OK. Took the injectors out and they all had a good spray pattern apart from number 3 which did not seem to have any fuel coming out at all. Took the injector off the end of the pipe and still no fuel coming out. Removed all the pipework, including the valve at the metering unit and everything seemed OK except that the rubber valve had quite a step in it where it had been sitting over the MU outlet hole. Put it all back together and primed the pipe with fuel and managed to get a good spray. Took the car for a run and the misfire had completely vanished.

I am thinking that the MU valve on line 3 is the problem but cannot explain why it would suddenly happen and result in a pulsing misfire that only occurred under 2,500 rpm. Very confused.

[JohnC]

It's possible there was a partial blockage in #3 somewhere between MU and injector poppet valve. You'll have cleared it when you took everything apart. Mind you, that doesn't explain why the misfire occurred under 2500RPM, unless you were using a more open throttle then. It also doesn't explain why you saw *no* fuel coming out when you took the injector off.

I assume that by "rubber valve" you mean the bung/barrel seal between the non-return valve and the MU. It sounds like that may have hardened as you note it's deformed. That may even have been the source of a bit of debris. In that case, your injection unit is due for an overhaul. Easy enough to do yourself, but requires scrupulous cleanliness (kitchen bench top!) and care, and you'll need to source the various seals and diaphragms. Unless you really want to DIY, you're probably best sending your unit off to a specialist. They'll be able to check the calibration as well, which is a job that requires specialist kit.

Having said all that, don't discount the possibility that you actually have an electrical problem (as well as a PI problem). If you spend enough time reading this forum you'll find that many, if not most, so-called PI problems are actually electrical.

Cheers,
John

[Mk2 Chopper]

It might be as simple as it required a bleed and now it's sorted.

When checking for pulsing injector lines make sure they are not touching each other or you could be feeling an adjacent line pulsing through the one you are holding.

I would see how you get on now you've cleared it. It's possible a bit of debris was the cause, just keep an eye on it. 

Gareth

[astontr6]

14 hours ago, Red6 said:

Did a 134 mile round trip this morning, the majority of which was motorway. First 67 mile leg was fine and the car cruised happily at 70 on the motorway in overdrive. The car sat for a couple of hours and when I came to drive home it had developed a misfire. Car would tick over with a slightly lumpy idle but nothing too drastic. When I tried to pull away there was a definite misfire up to around 2,500 rpm at which point it seemed to start running on all cylinders again. On the motorway it seemed fine unless I let the revs drop but on the A and B roads, at 2,000 rpm, the misfire was a steady pulsing, running OK for 3 seconds then misfiring for 3 seconds, running OK for 3 seconds and so on. When I got home I checked the injector pipes and they all had a reasonable pulse. Let the car cool down for an hour and then took the plugs out and they all seemed OK. Took the injectors out and they all had a good spray pattern apart from number 3 which did not seem to have any fuel coming out at all. Took the injector off the end of the pipe and still no fuel coming out. Removed all the pipework, including the valve at the metering unit and everything seemed OK except that the rubber valve had quite a step in it where it had been sitting over the MU outlet hole. Put it all back together and primed the pipe with fuel and managed to get a good spray. Took the car for a run and the misfire had completely vanished.

I am thinking that the MU valve on line 3 is the problem but cannot explain why it would suddenly happen and result in a pulsing misfire that only occurred under 2,500 rpm. Very confused.

Hi Red6!

I suspect that you had an air lock in that injector line Petrol can have a lot of dissolve air in it. If you are still using the original CAV filter system you should open the air bleed once a year to purge the air out. As the trapped air can travel down to the M/U and cause your sort of problem.

Bruce.

[Red6]

22 minutes ago, astontr6 said:

Hi Red6!

I suspect that you had an air lock in that injector line Petrol can have a lot of dissolve air in it. If you are still using the original CAV filter system you should open the air bleed once a year to purge the air out. As the trapped air can travel down to the M/U and cause your sort of problem.

Bruce.

 

6 hours ago, JohnC said:

It's possible there was a partial blockage in #3 somewhere between MU and injector poppet valve. You'll have cleared it when you took everything apart. Mind you, that doesn't explain why the misfire occurred under 2500RPM, unless you were using a more open throttle then. It also doesn't explain why you saw *no* fuel coming out when you took the injector off.

I assume that by "rubber valve" you mean the bung/barrel seal between the non-return valve and the MU. It sounds like that may have hardened as you note it's deformed. That may even have been the source of a bit of debris. In that case, your injection unit is due for an overhaul. Easy enough to do yourself, but requires scrupulous cleanliness (kitchen bench top!) and care, and you'll need to source the various seals and diaphragms. Unless you really want to DIY, you're probably best sending your unit off to a specialist. They'll be able to check the calibration as well, which is a job that requires specialist kit.

Having said all that, don't discount the possibility that you actually have an electrical problem (as well as a PI problem). If you spend enough time reading this forum you'll find that many, if not most, so-called PI problems are actually electrical.

Cheers,
John

Thanks John, my first thought was that it was electrical as I recently changed the plugs and coil. But now everything is working OK again with all the same parts!

I was very surprised to get absolutely no spray out of injector 3 but there was no obvious blockage or debris that I could see. At the very least I will replace the old non-return seals as I was not happy about having to put the deformed one back.

Richard

[Red6]

Thanks for all comments/suggestions.

Pump and filter have been replaced in the last two years and I am confident that end of the system is OK but there would have been some air in there at some point. Only item that is still original is the PRV but I did check the release pressure and it was within spec.

I will change the old MU non return rubbers and make sure all the air is out of the pipes. If the problem persists I will also put the old coil back in as it was only replaced as a precaution. Do the coils suffer from problems with heat soak from the engine? The problem only occurred after a long run and then being left standing for a couple of hours.

If all else fails it might be time to get the MU refurbished.

[Mk2 Chopper]

A coil tends to just stop working when hot if it's on its way out, like fuel starvation. 

Gareth

[Red6]

25 minutes ago, Mk2 Chopper said:

A coil tends to just stop working when hot if it's on its way out, like fuel starvation. 

Gareth

OK, thanks for the answer.