m50 missing and backfiring intermittently

[JGood]

I've got an intermittent miss/backfire condition. Sometimes it will just miss, other times I'll hear a backfire, sometimes it sounds like it's through the intake, other times it sounds like it's through the exhaust. Kind of hard to tell, as my exhaust is fairly loud and I have a cone filter which is loud.

Two things I've done recently that I'm thinking may have something to do with it:

-Ran out of gas
-Replaced the vanos unit

I know backfiring is usually a timing issue, so I'm thinking maybe something is wrong in there? But it didn't start doing this until 3k+ miles since I did the vanos swap. I thoroughly used the entire powerband for those 3k miles, no issues. So if it being caused by timing, something has come loose or broke, it's not the static cam timing that's wrong.

I ran out of gas, and I know that is very hard on the fuel pumps. I'm wondering if maybe I sucked up some rust or something into the filter or pumps? Or just burned out one of the pumps? I'll need to check this. But it wouldn't really explain a backfire, would it?

[pr0craztinazn]

My car started backfiring through the intake when I unknowingly ran low on fuel after my fuel level sending unit broke. If you have enough fuel, correct fuel pressure, and can verify your injectors are spraying fine, then maybe you have a different issue, like an intermittent ignition coil.

[328ijunkie]

You getting any CELs? Clean your injectors. I bet you pissed them off with Poo from the bottom of the tank, or tossed your FP and its loosing pressure randomly but im leaning towards injectors

[richardb]

I would say that it's not the timing because you say it's intermittent. Maybe a coil is starting to go bad? When I completed my first swap the car ran like you described and I checked the plugs- 1 of the 6 was sooty and black indicating a bad coil pack. Replaced the coil pack on that cylinder and it ran beautifully. I later replaced all 6 coil packs with the Hamburg Technic replacement coils.

[7pilot]

Does the engine miss at idle? You can locate a consistent missing cylinder with an infra red thermometer.
Pull the plugs and look for a sooty, or oily, or gas soaked plug.
The DME has some adaptive capability and may be compensating for an out of time cam but can only do this up to a point.

m

[JGood]

Thanks guys, I'll start by checking the plugs.

It doesn't do it at idle. It started by missing really bad only once. Almost like the car lost power for a full second, then came back. This happened once, then again maybe a week later, and it's been getting more consistent, but less severe. So, now I just get a little miss more often, as opposed to the big miss/backfire. Sometimes it will do it 3-4 times as I pull through 2nd and third gear, then not do it for another 10 minutes. It's very inconsistent.

[416stroker]

sounds like a week pump. if the cars losing all power for a second becaue the pump needs to play catch up. If its happening under acceloration.

I do what every one else said and also check fir a vacume leak.

[7pilot]

Do the symptoms worsen as the tank empties?
If so, I would start by checking lift pump, and changing the fuel filter. Carefully empty the old one into a container via the intake side of the filter, to see what and how much sediment is in it.

m

[JGood]

Do the symptoms worsen as the tank empties?
If so, I would start by checking lift pump, and changing the fuel filter. Carefully empty the old one into a container via the intake side of the filter, to see what and how much sediment is in it.

m

Nope, not at all. I can run it to fumes, and it will do it the same as fully filled.

One other thing, it only seems to do it between idle and 4k or so. If I'm full throttle, it won't do it above 4k. The car still pulls hard up top. Sometimes I can go thirty minutes of driving without a single miss. Other times it will run like it's almost out of gas... missing every few seconds. Overall, it's really not a huge noticeable problem, but it's bothering me, and sometimes passengers can notice.


Still haven't had time to check anything. Hopefully Friday.

[graphikg]

I think I might be having somewhat of the same problem - except, when I even try to give it half throttle the car shuts off completely. I would tell you everything which people have been telling me... Check your coil packs, and fuel filter. My problem is probably graver than yours due to the way it shuts off along with the loss of power. I dont even know how it does up top because I cant even get there in RPMs.

[JGood]

Replaced fuel filter.

Verified in tank pump is working.

Verified external pump is working.

Checked all grounds.

Verified there are no vacuum leaks (to the best of my knowledge, anyway).

Checked plugs, all seem to be normal, slight off-white color. I guess that's normal?

Swapped Main and O2 relay's. Didn't have a spare fuel pump relay to try.


Still no go.


BTW, the fuel that came out of the old fuel filter was crystal clear. Especially clean for a 30k+ mile filter. And the pickup screen on the in tank pump wasn't very dirty.

[e30s50dan]

Have you tried the infra red thermo on each exhaust port. I did and managed to have two weak coil packs. Replaced and ran good, till I sold it. Good luck

[JGood]

Can you describe in a little more detail? What am I looking for, what is the procedure, etc...?

I can definitely pick up a spare set of coils to try.

[xwill112x]

have you removed the injectors at anytime?

[e30rapidic]

if you have a spare set of coils to try....definitely do that, at least to rule them out if it doesn't fix it. I'm wondering if you have a bad plug still. I was chasing a misfire in my Ford for a long time, turns out it was a plug that had less than 15k on it.

It was bad enough at times that I had my OBDII diagnostic tool plugged up continuously to reset the CEL and watch engine parameters.