740 i runs rough until warm

MarkGBC · 01-03-2007, 05:39 PM

I have a 1993 740i that when started in the morning it is hard to keep it running. It runs very very rough and you have to keep pumping the accelerater pedal to keep it running. Once the motor is warm then it idles very nice at around 7 or 800 rpm and for the rest of the day it starts and idles just fine. Is there a start up senser that could be bad?

muleman · 01-04-2007, 09:37 AM

What diagnosis work have you performed to this point?
My 1994 740il gradually began exhibiting the same conditions you describe, which seemed to worsen when it was cool and damp. I spent a good amount of time checking the usual suspects, vaccum lines, fuel pressure, etc. Then I pulled a spark plug even though I had only 40K miles on Bosch platinums. They were pretty badly corroded so I change them out and viola, fires right up everytime even after sitting for several days.

MarkGBC · 01-04-2007, 10:31 AM

The plugs were dirty but not that bad. Once the motor warms up it idles perfect the rest of the day. It isn't that it doesn't start right up because it does. It is more the idle once it starts. It doesn't seem to lack fire. If I warm it up for 1 minute then shut it off, I can start it 2 minutes later and it idles fine. It seems to have something to do with the temp of the motor.

ZONDERVANUS · 01-11-2007, 02:57 PM

Mine does the same thing and then the CHECK ENGINE light comes on and it wants to shut off...

I was told that my '94 740IL needs a Cold Air Start which is the silvery finish tube in front of the motor...Its been chilly in Miami this past week.

My tranny also takes a little bit to engage when i put it into reverse.

lincolnmaster80 · 01-17-2007, 07:58 PM

You might try all the seals and gaskets on the intake side after the MAF. Might be an intake gasket cracked causing false air. Not an unfamiliar problem on the E32 7-series and 80-90% of the time that's the cause.

ZONDERVANUS · 01-17-2007, 10:44 PM

Quote:

Originally Posted by lincolnmaster80

You might try all the seals and gaskets on the intake side after the MAF. Might be an intake gasket cracked causing false air. Not an unfamiliar problem on the E32 7-series and 80-90% of the time that's the cause.

Thanks I will look into that with my mechanic... I heard the E32's were problematic also with electrical problems...

lincolnmaster80 · 01-17-2007, 11:45 PM

To be more specific, when it's cold, the gaskets aren't sealing properly. When it heats up, the gaskets expand to seal the false air leak. So even if you do inspect the gasket and is found to be visually ok, this just might be the problem.

FRED_C · 12-04-2007, 12:24 AM

I also have the same problem with my 94 740i. Check engine light comes on and rough idle in the morning, then after about 10 minutes of drive time its fine for the rest of the day. Curiously, the car was fine all summer (The average temp here in Palm Desert during the summer is 110-115, cooling down to about 99 by midnight). But as it got cooler when fall came, the problem started happening again. I've replaced the MAF and the "silvery" idle control valve. I also replaced the throttle body boot and checked for gasket leaks as recommended and replaced the air filter. All the plugs are new and I have individually grounded each coil. Still no help.

There is an intake air temperature sensor (The #2 O2 sensor, I believe) located on the forward passenger side of the intake manifold. Could this be the source of the problem and/or is there any way to test it? Schematics reveal that it is second in line electronically to the MAF. This is the least expensive of the 4 O2 sensors but still I'd rather not have to spend another $50 if the problem lies somewhere else.

Thanks for your help.

BAVARIANM · 12-05-2007, 02:28 PM

The first thing to check on that engine is the casting number. It is located on the right side, next to the coolant drain plug. If the casting number is 1 725 963, or 1 742 998 you have a Nikasil block and the problem is cylinder wear. Nothing you do will correct that problem except an updated engine.

The other thing to check is on the back of the intake there is a square plate. The diagphram in the valve may very well be cracked. Spraying starting fluid around the plate should reveal that

FRED_C · 12-06-2007, 01:56 PM

If the square plate you're referring to is the Emissions Vent Valve, then yes, I replaced that about a year ago. I'll look for the casting number this afternoon, but do you think cylinder wear would apply even considering the cars runs absolutely perfect during the hot summer months? The check engine light and rough idle is only an intermittent problem when its cold outside.

lincolnmaster80 · 12-06-2007, 02:06 PM

Quote:

Originally Posted by FRED_C

but do you think cylinder wear would apply even considering the cars runs absolutely perfect during the hot summer months?

Yes. Heat is your best friend when it comes to something like this. The cold tends to shrink the gaskets and seals, also making them stiff and nonconforming if you will. There's enough compression to keep the car going but not enough to make it run smoothly. During the summer, everything is fine because the heat keeps everything loose and tight. Since it's only during the cold you are experiencing this, then this is most certainly a plausable problem.

For instance, my car has a hunt idle on start up now that the winter is upon us. Only when it has sat for a while and cooled down. The intake plenum gasket is cracked somewhere and causing a slight air intake leak that the engine almost can't adjust to quickly enough so the RPM tends to hunt between 400-1000 RPM, sometimes it stalls. Once it's been running for a bit, I can start it up without any hunting. Summer time is fine.

Amazing the problems cold weather can bring out...

POST REPLY TO THIS THREAD