The Stumbling M20 Engine

[Quinthirty]

I have used the car as DD for a year now, and was hoping that after having it tuned or diagnosed by "qualified" BMW mechanics and letting it run for awhile, things would have normalized. She was a garage queen, and now she runs much better than when I first got her. M20B25 is bone stock.

Problems: Rough idle. Sometimes, she doesn't start in one go.

When I start her cold, before she only took one click to start. But lately, she's acting up. Sometimes, it would take 2, 3, or 4 cranks to start her up mostly when she has already run or parked a few hours. I'm not sure if its even related? I am unsure if I can clean the CPS though, if that would even help, or damage the sensor.

Idling has a stumble, hot or cold. Runs smooth for a few seconds, and stumbles but there is no fluctuation on the tach though. She's steady at 800 RPM with the A/C on, 650-700 when off. She has a slight uneven knock at the front of the engine sometimes, and it seems that its coming from the timing belt side. One mechanic noticed this too and recommended that we look at the tensioner (but I don't think its related either, but I could be wrong). Or should I check if timing belt missed a tooth?

Its an A/T and pick up is sluggish on lower gears. I've seen a good number of videos on youtube, and I can say that for the same stock engine, mine is quite sluggish compared to a lot of them. I haven't tried to redline the engine as I'm unsure about its condition especially when I saw that the PO re-used the torx bolts (cheapo)! She runs very well when cruising at 55 and above though...smooth.

These is what I have changed so far:

1. AFM (remanufactured)
2. Ignition coil (a stupid mechanic killed it)
3. Spark plug wires
4. Valve adjustments
5. M50 injectors
6. Chip
7. Vacuum hoses except the ones that goes to the throttle body / intake manifold.

I've asked the mechanic to check fuel pressure & he said that pressure holds up around 40+ psi at the rails.

I have replaced the plugs and oils and filters twice in a span of a year, but was told that I don't need to have rotor and cap replaced any sooner, though I have spare lying around.

The suspects:

1. Oxygen sensor I guess that this is the only part that I'm left guessing that I have not replaced, I'm likely to replace this soon, but its still a guess. Error reading is 1222, so this part is a likely candidate, or a vacuum leak. If this is the case, I'm surprised that none of the mechanics recommended O2 sensor replacement. I'm buying this soon and will replace it myself.

2. Compression? Again, I'm guessing. I'm unsure of the readings done by the mechanic as it showed cylinders 4 & 5 at around 120 psi, while the rest were at 140ish to 150. Mechanic did not do a wet compression test nor leak-down test. At any rate, the car does not blow smoke nor consumes oil, so I'm confident that the compression should be within acceptable levels (less than 20% difference). However, I'm still unsure whether this can contribute to the rough idle.

Sorry for the long post. I just want this baby to run smoothly and would appreciate some diagnostic recommendations or solutions to get her fixed.

Thanks again, R3V.

[bmwstudent]

Why do you have m50 injectors? Check or replace the vacuum lines gong ti the throttle and manifold. And yes it is possible with that code that it could be your o2 sensor, good luck!

[Quinthirty]

Why do you have m50 injectors? Check or replace the vacuum lines gong ti the throttle and manifold. And yes it is possible with that code that it could be your o2 sensor, good luck!

17lb injectors are 4 pintle and supposedly have better spray patterns compared to stock. Those vacuum lines you mentioned are new. Thanks.

[jlevie]

A bad/aged O2 sensor (replacement interval is 100k) and intake leaks are probably the most significant cause of the engine's problems. Replace the O2 sensor and have a shop run a smoke test to find the leaks.

[Quinthirty]

Thanks, Jim. I'll do that.

[nrubenstein]

I would also clean the CPS. No, you can't harm it with a bit of solvent and that also can create real running problems.

[603Racing]

17# injectors (m50) can cause rough idle/running if you don't have a chip to go with them. That may not be your cause, but it's a possibility.

Always suspect possibly vacuum leaks in these situations. Which I had a $1 for every time Jim suggests a smoke test. :) There's a reason for that too, M20's and M30's like to leak.

Another good thing to check is the round electrical plug under the intake/throttle body. This can become corroded and cause all sorts of odd issues that mimic other failures.

Check the vacuum line running from the FPR to the intake. You stated you were running good fuel pressure, but check it anyway, takes 10 seconds. If that vacuum line is cracking then replace it, if it smells like fuel or is wet then replace the FPR.

[jlevie]

17# injectors (m50) can cause rough idle/running if you don't have a chip to go with them. That may not be your cause, but it's a possibility.

17lb injectors are close enough to the stock injectors that the DME can learn how much less fuel to inject for proper operation if the O2 sensor is good and there are no other engine management problems. That won't work with larger injectors because the available fuel trim is limited and the DME can't fully correct for the increased flow.

Always suspect possibly vacuum leaks in these situations. Which I had a $1 for every time Jim suggests a smoke test. :) There's a reason for that too, M20's and M30's like to leak.

M20/30 engines aren't unique in the this respect. Any engine with electronic fuel injection is extremely sensitive to unmetered air. And modern engines have lots of places to leak.

[IAmGary]

I would also clean the CPS. No, you can't harm it with a bit of solvent and that also can create real running problems.

I agree. Overlook this at your own peril: you can throw parts upon parts at this kind of problem and never touch it if the CPS is dirty. This was my problem: car was burning oil, eventually developed a misfire all because of a dirty CPS. It's one bolt to remove, clean it with a rag and put it back on.

Photo of it's location:

[Quinthirty]

Thanks for the tips, gents. I'll clean the CPS.

I did the injectors-and-chip combo. I have a spare 173 stock ECU though, and I'll probably replace it now that Jim mentioned that it can run with the stock DME. It'll save me some gas money too, as the chip requires higher octane fuel or it knocks like crazy.

I've also replaced the vacuum hose from FPR to manifold. That is one of the 1st ones I replaced when I got the car as I noticed that the hose was quite old. All other hoses i.e. to ICV, valve cover are all good. The only ones that are left are the "complicated hoses" on the throttle body, so if the rough idle doesn't go away after I replace the O2 sensor, that will have to be replaced too.

I'm not sure if the brake pressure is also related as I notice sometimes when I'm in a stop and my foot is resting on the brakes, the pedal goes all the way down. But when I step on it again, the brake pressure is back. I was told that to test which one has a leak, turn off the engine, and pump the brakes. If the pressure holds, its the hydro vacuum, or if it releases pressure, its the master cylinder. I'm not sure whether that's a good diagnostic test though.

[Quinthirty]

BTW, can the sensor take contact cleaner / or carb cleaner, or those solvents are too strong?

[Skunkworks]

yes it should be fine.

[Quinthirty]

Just finished cleaning the CPS and it was such a PITA to take out. The allen nut wasn't cooperating so I gave it some WD-40 love... Meanwhile, I cleaned the CTS sensors too before I took it out. Anyway, the CPS was covered in thick gunk and grime indeed.

On cold start, I had smooth idle, finally! But it took 3 attempts to start (???...fuel pressure?). I'll observe the idle in the next couple of days especially on closed loop. If it still goes rough especially when hot, then the O2 sensor is it. Just waiting for shipment to arrive...

[bmwstudent]

Congrats, it's always nice to have something go well.

[sweet3]

Glad to hear you sorted out one of your problems. Your other problem could be the cylinder head. Does you car smoke/steam out of the exhaust? It is common for bmw heads to crack from overheating. When you turn the car off, the water pressure will cause some water to enter the piston chamber. This could be the reason for a tough start. I had this issue, but mine was because of a bad gasket.
Goodluck!