The Stumbling M20 Engine

[jlevie]

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 said that the DME can adjust to those injectors, not that it was a good idea. Nor is there any performance advantage to larger injectors unless some serious work has been done on engine internals to significantly increase mass flow through the engine (or FI). The stock injectors (if clean and working properly) are more than the engine needs as is.

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.

All hoses on an E30 (including the cooling system) should have been replaced twice by now (every 10yr/100k), but the odds are that none have been replaced. Best practice is to just replace all or the hoses. Then have a smoke test run to check for leaks elsewhere.

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.

The pedal sinking when you hold pressure on it is either a leak or a bad master cylinder. Get the car up in the air and check each caliper and all lines for leaks. If none are found, replace the master cylinder. When replacing the master cylinder, use a a bit of rag on a wire to check the booster for brake fluid that may have leaked from the master cylinder. If there is brake fluid in the booster it will need to be replaced also. Brake fluid will destroy the diaphragm in the booster pretty quickly.

[Quinthirty]

Car is idling well now. But I'm still stoked by the hard start sometimes. It took me three clicks yesterday after stopping for an hour. This morning's cold start also took 3 attempts.

I almost run the tank to empty last week because of bad traffic, and I think that my fuel pump sucked the dirt from the bottom of the tank. Anyone knows how to clean the pumps? I don't want to take it to a shop.

[Quinthirty]

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!

No smoke, no overheating, no milkshake either in the water...and this baby sits in traffic much like the 405 daily. I think this a rebuild scenario you're talking about but I don't think this engine is close to getting there yet. Thanks.

[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!

[bmwstudent]

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

[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...

[Skunkworks]

yes it should be fine.

[Quinthirty]

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

[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.

[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:

[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.

[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.

[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.

[Quinthirty]

Thanks, Jim. I'll do that.

[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.