M20B25: backfiring and won't rev

[e30austin]

I have an 02/1988 325i cabrio in that needed an engine replacement. I changed it with a known good engine out of a parts car that I had, an 01/1988 325is. The engine ran spectacular when pulled, with no issues or other funny business.

I installed the engine and got it running. Immediately, I knew something was up. It hardly ran on its own. After some quick diagnostic work, I realized cylinders 1 and 6 had no fuel being delivered to the chamber. After swapping the fuel rail w/ injectors and pressure regulator, off the old engine, the idle smoothed out and it was firing on all six, as it should. I hooked up the AFM and went to to take it out for a test drive. It was then, that I fired it up and it promptly died. It wouldn't run with the AFM plugged in. It also won't rev past 1/8 throttle. It backfires through the intake (anything past 1/8 or 1/4 throttle) and bogs down, to the point of being undrivable, obviously.

Here's what I've done.

-new set of Bosch 7909 (WR8LC) to replace the platinum plugs someone installed. No change.

-swapped AFM from a known good, running vehicle. No change.

-unplugged TPS, CTS (blue), ICV and 02 sensor. No change in idle or rev-ability.

-tested fuel pressure (at the rail) and it is within spec. 48lbs with vacuum hose unplugged, 43lbs with hose plugged in, and promptly increases to 50lbs when opening the throttle, with the hose connected (pressure goes up, even when backfiring, so I can say it isn't a fuel delivery issue).

-smoke tested, no leaks anywhere.

-swapped distributor cap/wires, no change.

-new CPS, no change.

-swapped front crank pulley, no change.

-tried a 380 DME from a known good, running vehicle, no change.

-inspected fuel injector harness and its connector, no issues, applied some dielectric grease. No change.

I'm close to out of options. My next project will be assuming it has a faulty engine wiring harness, at this point, but I don't really want to change it, just for the sake of changing it.

Any advice or suggestions would be much appreciated.

Austin

[325e '87]

Man, your experience is so far out of my league but I understand how and why you’ve done the above list. I didn’t see that you confirmed the timing marks, which I know, the engine ran fine before... but, if there’s ignition timing that allows backfires, what else could it be (besides the cps)? I mean, it’s just not physically possible for the timing to be that far off unless the dme fires the injectors at the wrong time, and the spark hits at the wrong time?

Plug wires mixed up? Bad ground somewhere? Leaky injectors?

[e30austin]

Man, your experience is so far out of my league but I understand how and why you’ve done the above list. I didn’t see that you confirmed the timing marks, which I know, the engine ran fine before... but, if there’s ignition timing that allows backfires, what else could it be (besides the cps)? I mean, it’s just not physically possible for the timing to be that far off unless the dme fires the injectors at the wrong time, and the spark hits at the wrong time?

Plug wires mixed up? Bad ground somewhere? Leaky injectors?

I did change the timing belt, due to age, and spun the engine over ten times to confirm timing was dead on (I always do). That was immediately my first thought. It idles too well to be off time (smooth as butter).

Firing order is correct.

Possibly a bad ground, but doubtful, since the M20 only has the two - oil pan to frame rail and the one coming off the engine harness at the passenger side shock tower.

I don't think leaky injectors would cause it to completely sputter and backfire. It acts like it has a massive vacuum leak, but it definitely doesn't :/

[varg]

Did you check the ignition timing with a light?

[e30austin]

Did you check the ignition timing with a light?

Yes, I did. Timing is dead on (tested #1 cylinder) and the DME does what it's supposed to, when you open the throttle (advances/retards timing as needed).

Update from today...

Performed a fuel volume test, it has plenty of volume (in case someone was thinking clogged fuel filter).

As a last resort measure, I swapped the engine harness assembly (w/ injector harness) off the previous (original) engine to the customer's car. Nothing. Same issue. I pulled the upper timing cover off and verified the upper mark at the cam gear, and the lower mark at the harmonic balancer/timing cover were dead on. They were.

I am 100% out of ideas at this point. The customer wants his car back and I want to get paid. Any suggestions are appreciated.

[MrBurgundy]

Wow, that sounds like a nightmare..

Umm this is probably not your case since you said it was runing well before, but I jsut had a car in that had the same shit happening to it and what i found was one of the rockers wasn't opening the exhaust valve so when the intake valve opened all the exhaust would shoot out through the intake

[HerbE30]

Which brand CPS and is the gap correct?

Have you tried testing the sensors with a multi at the DME?

[uturn]

Meowwww

[MrBurgundy]

figure it out yet?

[325e '87]

Wow, that sounds like a nightmare..

Umm this is probably not your case since you said it was runing well before, but I jsut had a car in that had the same shit happening to it and what i found was one of the rockers wasn't opening the exhaust valve so when the intake valve opened all the exhaust would shoot out through the intake

Yeah, I’m not a pro but damn if I aren’t curious. Seems like it HAS to be something physically reversed or broken. The description is something that’s impossible unless something is backwards.

[e30austin]

figure it out yet?

Nope, nothing. "Uturn" suggested a clogged catalyst, which I considered to be a possibility, but disconnecting the downpipe made no difference. Has 165psi on all six, so I don't think it has a valve lash issue.

Frustrating. I've changed so many M20s over the years, with the occasional hiccup, here and there, but this is bullshit!

[AWDBOB]

I am so sorry- I had this exact issue recently (even down to the no fuel on 1 and 6).

In my case, it was the C191. I know you said you changed it and whatnot, so that really isn't much help, but it's creepy how similar my issue was to yours.....backfiring, fueling, etc etc. Different in that your rail/inj change fixed the firing, though.

Does the car throw any codes? Mine threw a coolant temp code even though the issue was (kind of) unrelated.

Edit: I went through two injector harnesses before I got one that worked (the third), and both of them were taken off of good running cars. As dumb and redundant as it sounds, maybe try swapping the inj harness again. I've just had so many issues with them recently, all symptoms just like the ones you're describing.

[ezlnh8u]

Was this ever resolved?