Cold Start Problem M40 1990 316i

[omera60]

Hi All,

I am having an issue on the cold start where the car starts fine but if I touch the throttle I sense a hesitation and rpm drop for a few seconds. The car won't stall or anything. It only happens on a cold start and will last maybe 5 mins max after that drives beautifully. If I don't touch the throttle everything's fine. I can put it in drive etc (auto) no problem at all. I don't have a vacuum leak, did the smoke test, nothing came up. Emissions seem ok (non-KAT) Measured all the cables resistance all ok, AFM and blue plug temp values match. Done all of the following items, replaced or refurbished them;

-Timing belt change
-Brand new AFM
-New High Power spark plugs
-New fuel filter
-All new fuel lines
-All new oil return lines
-New injectors
-Changed both blue and brown coolant temperature sensors
-Brand new radiator
-Ran smoke test, no leaks from it
-Brand new coil
-Brand new battery
-Brand new ICV
-Checked TPS
-Brand new fuel pump
-Brand new distributor cap and parts
-Refurbished alternator
-Brand new DME and Fuel Pump Relays
-New Exhaust

No faults are stored. Tried the 10 min battery disconnect DME reset, nothing. Compression on all cylinders is perfect. If I press the pedal a bit more (without babying it) the hesitation doesn't occur or cannot be felt. Any theories?

Best,
Omer

[roguetoaster]

How's the camshaft? It's possible that it's worn and not generating proper valve lift which the car cannot compensate for when cold.

The whole cam kit isn't expensive and was a worthwhile thing to do on one of my former M40s.

[omera60]

How's the camshaft? It's possible that it's worn and not generating proper valve lift which the car cannot compensate for when cold.

The whole cam kit isn't expensive and was a worthwhile thing to do on one of my former M40s.

Hi roguetoaster,

thank you very much for the reply. The car is currently at 58k km’s do you think that it is possible for it to occur this early? I have changed the tappest about a year and a half ago, back then there was a discussion about the cam but I was told it looked fine. Nevertheless I was planing to open the cylinder head cover to do an inspection so I ordered the parts yesterday once they are here I will open it.

I will be renewing the heater element o-ring below the throttle. Our family mechanic told me that before when he unplugged the ventilation hose that is connected to the cylinder head cover there used to be a vacuum once he put his hand on the hose now there isn’t any.

Also tomorrow I will be going to a Bosch Car Service to get a Carbon Clean. Will report back on it later on.

[omera60]

Hi,

Just to report back. I did inspect the camshaft with my mechanic, it was in perfect condition. The carbon clean did miracles though. It made such a big improvement. The acceleration, shakiness etc all gone. We also changed the O-ring of the heater element which was problematic. Now I’m waiting to see if there is any improvement during morning. I would recommend a good carbon clean to everyone if not done before.

[roguetoaster]

Ah, didn't realize that the car was such a low mileage example. Most M40s seem to round over the cam lobes in the 150-180k range.

Will be interested to hear your findings from the decarbonization process during a cold startup.

[omera60]

Ah, didn't realize that the car was such a low mileage example. Most M40s seem to round over the cam lobes in the 150-180k range.

I will be interested to hear your findings from the decarbonization process during a cold startup.

So I can say that there is a very noticeable difference during the cold start. The issue is almost eliminated, I could still feel a little hesitation when I give it throttle. There is a huge improvement in the way it accelerates though, it is smooth as silk once you get going.

On the cold start issue, I also tested the fuel pressure and fuel delivery both seem to be in spec. Any more ideas what may be the source of mysterious hesitation?

[omera60]

As many of you know we have a lot of time in our hands right now so I decided to go all the way on this issue.

After checking various sensors, performing diagnostics, etc. I can't for sure tell you anything. I checked all the wiring from the DME side to confirm all the Ohm's and volts, there is no issue there. I changed the DME for a working one just for the sake of it and the problem is still there. I tried to reset the car via 10min unplug negative terminal attach it back let it idle for 10min and go for a drive, still no change. The only noticeable thing I have found is if I unplug the AFM and then start it (emergency program on the DME) the issue is not present. But the whole AFM is brand new from BMW and the issue was present before the AFM change as well. Checked the pinout of the AFM everything is in limits. Attached the car to 5 Gas analyzer currently, the CO volume is 0.91. Opened up the factory repair manual it states for the warm car at idle to have 0.7-+0.5 CO. So my car is in limits. I have performed a smoke test before at the dealer while the car was warm no leaks. Tested the compression on cylinders while cold. All 4 of them are perfect, 10-11 bar. Also got the OEM 1k Ohm F8LCR plugs which when tested are in fact, 1k Ohm. Compared the temp values sent to the DME from both the blue plug and the AFM temperature from BMW's diagnostic system when cold, they both match up. checked all the valves fro any buildup, clean as new. Any ideas about what may be the cause of the issue?