Misfire issues - injectors? Gurus needed!!!

[shrike071]

I am deep into deciphering the cause of my S52's misfire. It is completely random - and does not seem to hit on the same cyl. Sometimes it'll go for a few seconds without a miss, and others - it's like a popcorn popper for a few seconds... All over the map. No load, under load - no change. I thought I had it solved, but it is still there. The list of things I have done is as follows:
- replaced crank & cam position sensors, replaced fuel filter, checked MAF, pulling a rock-steady 20in of vac at idle, changed plugs, replaced coils, replaced O2 sensor, changed DME, changed chip, no oil in coolant or vice-versa, and I am sure I am forgetting some other things.

The DME is giving me the 1281 code but I suspect that is from swapping out the DME, and the emissions show a high HC.

The odd thing about this misfire, is that I once did the 'shogun reset' of the DME, and when I initially started it, the car ran beautifully. Nary a miss to be felt. I thought "Eureka!", only to be let down when the miss re-appeared when the car reached operating temp. WTF!!!!

I started to think of all the things that change in the car when it warms up, and I was wondering if a bad injector would do this. Does the ECU change the mix when the car is cold? If so - would it be possible that it does fine shooting in the richer/colder mix, then starts screwing up when warm? Also - would the injector cause a high HC in emissions? If it's missing due to an injector (or injectors), does the injector simply not fire (causing a miss), or would it dump a ton in there and cause that cyl. to 'flood', thereby causing a miss and the high HC?

I am completely and utterly stumped here....

[Jon K]

The ECU does run it rich during cold. Not super duper crazy but like 13.5:1 is typical. In your list of things you fixed, I do not see fuel pressure or fuel pressure regulator being tested. I would check that. If you need, I have the injectors from my 1992 525 (original) that you can try and mail back.

[Ferret]

The list of things I have done is as follows:
- replaced crank & cam position sensors, replaced fuel filter, checked MAF, pulling a rock-steady 20in of vac at idle, changed plugs, replaced coils, replaced O2 sensor, changed DME, changed chip, no oil in coolant or vice-versa, and I am sure I am forgetting some other things.

You could well have a leaky injector, have you done a compression test and checked the plug leads if fitted?

[shrike071]

Have not done a compression test, but that's on the list. Really not suspecting anything here since it ran perfectly when I did the reset, and frequently runs perfectly when cold.

I'll check the plug leads - but they're coil-on-plug, and all new.

If I run the M50 injectors in an S52, won't that be cause for big-time leanout?

[Ferret]

Have not done a compression test, but that's on the list. Really not suspecting anything here since it ran perfectly when I did the reset, and frequently runs perfectly when cold.

I'll check the plug leads - but they're coil-on-plug, and all new.

If I run the M50 injectors in an S52, won't that be cause for big-time leanout?

You can physically check to see if the injectors are leaking or not firing properly... if you're as mad as I am that is. I checked my injectors by pulling the rail, turning it upside down and kicking the engine over. (I told you I was mad.)

You'll immediately know if one's not firing properly, and it'll pressurise the rail so you can see if one of the injectors is piddling fuel all over the place.

[Ross]

Wow, that's a long expensive list. The sheer randomness you describe makes me say it's going to be electrical.
A corroded connection, or loose wire maybe. Can you precipitate the problem by fooling around with any of the wires?
Like Jon K says the pressure regulator(or the vac line) should be checked but I don't see it being intermittant.
Sometimes you just hope for a complete failure don't ya?

[shrike071]

Wow, that's a long expensive list. The sheer randomness you describe makes me say it's going to be electrical.
A corroded connection, or loose wire maybe. Can you precipitate the problem by fooling around with any of the wires?
Like Jon K says the pressure regulator(or the vac line) should be checked but I don't see it being intermittant.
Sometimes you just hope for a complete failure don't ya?

Yes - and the expenses keep going up. I am devoted to solving this problem. I am of the same opinion as you that it seems to be an electronic issue, but I just plunked down another $300 on new injectors. We'll see what that does. After this, I'll be swapping the chip from Turner for a new one.

And a complete failure would be most welcome at this point.

[Podmore]

I'm thinking it could be a fault in the induction wiring in one or more of the injectors - higher resistance when it gets warm could lead to the injector electromagnet not pulling the piston up to open the injector properly. From your list above it seems it's about the only thing not changed yet - hope your $300 for the new injectors proves well spent!

[Bin_jammin]

I've been a mechanic for a long time now, and a trick I like to use when diagnosing a skip is to use mechanic's wire. Bend some mechanic's wire to a few inches long, with a j-hook on the end. Now double back the j-hook. I don't know how to describe it better, but basically what you want to be able to do is have the piece of wire protruding from under the plug wire boot, but still contacting the terminal in the plug wire.

Step two is to take some type of jumper lead with an alligator clip on one end, and clip it to a good ground somewhere on the car or engine.

With every cylinder wired up like this, you'll be able to start and run the car, and use the ground lead to individually short out each cylinder to ground. It won't harm the car. Each cylinder will drop out individually when you give it ground (you'll see the spark jump the air gap) and your engine will have a much more pronounced skip as that cylinder has gone dead.


The point is that when you find a cylinder that you've shorted that doesn't result in a change of power, or affects the running of the car less than the other cylinders, that's the cylinder that has the problem. Finding which cylinder is the cause of the problem is the first step to diagnosing what the problem is.

If you don't understand what I mean (and I won't blame you, as I tend to ramble) just let me know and I'll try to explain better.

[Ferret]

I've been a mechanic for a long time now, and a trick I like to use when diagnosing a skip is to use mechanic's wire. Bend some mechanic's wire to a few inches long, with a j-hook on the end. Now double back the j-hook. I don't know how to describe it better, but basically what you want to be able to do is have the piece of wire protruding from under the plug wire boot, but still contacting the terminal in the plug wire.

Step two is to take some type of jumper lead with an alligator clip on one end, and clip it to a good ground somewhere on the car or engine.

With every cylinder wired up like this, you'll be able to start and run the car, and use the ground lead to individually short out each cylinder to ground. It won't harm the car. Each cylinder will drop out individually when you give it ground (you'll see the spark jump the air gap) and your engine will have a much more pronounced skip as that cylinder has gone dead.


The point is that when you find a cylinder that you've shorted that doesn't result in a change of power, or affects the running of the car less than the other cylinders, that's the cylinder that has the problem. Finding which cylinder is the cause of the problem is the first step to diagnosing what the problem is.

If you don't understand what I mean (and I won't blame you, as I tend to ramble) just let me know and I'll try to explain better.

He could just try unplugging the individual LT looms :p

[attack eagle]

SO what checks does the DME run when it reaches operating temp? what systems does it turn on via a vacuum solenoid? EGR maybe?


Goes to a different map, goes to closed loop, starts reading o2 info.