525i E34 Runs rough when warm after overheating

[Marcusd]

Hi, I wonder if anyone could give me any pointers on this one?

A couple of weeks ago my plastic waterpump impeller failed (at 247K Miles, so it did well really - I thought I had replaced the pump on both of out E34's but obviously missed this one!).

2 weeks on with a new metal impeller water pump and 600 miles later the car overheatyed again in traffic and blew the top coolant hose off the thermostat housing resulting in a lot of steam and spilt coolant. (This happened right at the top of the Dartford Crossing, for those in the UK who know the place!). I managed to refit the hose and fill the coolant with water and premixed washer fluid which we had in the car. This was enough to get me through the Dartford Crossing and onto the hard shoulder through the toll booths to wait for recovery.

When we got home the car would not start so was left overnight. The next day it started but was very lumpy. I took off the coil packs to find the plug wells had a fair bit of coolant in them so removed the coil packs and blew out the majority with a compressor, then removed the plugs and mopped up the rest of the damp with a rag on a metal rod.

The car started and ran fine after this so I set about working out what caused the overheat.
It turns out the Aux fan was seized so a replacement has been fitted and the fuse changed. The fan works fine now on low and high speed.

The issue I have now though is that the car runs fine from cold but as it warms up it gets progressively lumpier. It ticks over OK but starts to misfire if you raise the throttle. When properly hot it is barely drivable.

I cannot see any water in the oil or oil in the water so am hoping it is not head gasket. Due to all the water and steam which covered the engine I am wondering if a sensor could be the cause. Has anyone any ideas or pointers please?

Cheers, Marcus
2 X '95 E34's 525i M50
Ashford, Kent, UK

[whiskychaser]

You are a brave man getting out of the car on that bridge! I dont even like driving over in the nearside lane:D
TBH I dont think the aux fan was the original problem. Mine sticks and the engine will run all day without it- I just need to turn the air con off in standing traffic. The fact it blew the top hose off suggests too much pressure in the system.
I'd whip the plugs out again and see if any are wet. While they are out I'd spin the starter to see if any water comes out of the plug holes. And of course do a compression test. IMHO the sensors likely to affect it when its warmed up are the temp sensor and O2 sensor. Because of its position and where the hose blew, I'd say temp sensor is most likely. Maybe the connections just got soaked?

[Marcusd]

Looks like good news. After waht you posted, I took off the coil pack again and removed the plugs. No water came out of the plug sockets when I turned the engine over and the plug electrode ends were dry. I gave the plugs, sockets, and coils another dry as there was still a lot of condensation on them. The coil primary resistance was checked and all were 0.9-1.0A (Bentley says 0.4-0.8, but my meter is only a cheapie and I was happy enough that all read the same). I cleaned out the air intake and filter plus sorted a couple of vacuum leaks which seem to have appeared on hoses by the throttle body entrance, which were sorted.

Took it for a drive and all seems OK now. I have ordered up some new hoses as the old ones were cooked and ordered a viscous fan clutch.
The car does not spend a lot of time in traffic and I think that the last couple of breakdowns were just showing weaknesses in the cooling system. 2 weeks earlier the water pump failed then last week I had the second overheat and the hose blow off. This hose could have just been because I didn't tighten the clip enough after fitting the waterpump but, by this stage the car was already hotter than it should have been.

I suspect that the combination of the aux fan being siezed and the viscous fan clutch being very weak caused the overheat - this was the first time the car has been stuck in a 45 minute jam for a few years and the cooling system could not cope.

The viscous fan clutch is so weak that I can stop the fan easily and it carries on spinning a for a lot longer than on out other E34 when the engines are turned off.
This one has done 247K miles now so I suppose the cooling system was due an overhaul anyway. The lucky thing is that I seem to have caught it before any head damage has occurred. (Still going to carry a bottle of K-Seal in the car though, just in case!)

[whiskychaser]

Glad you got to the bottom of it. Link to test fan clutch if you still want it:
http://www.mwrench.com/Whitepapers/FanClutchtest.pdf
When 'er indoors says 'Come and look at this dress' for the tenth time today, it will give you a reason to escape:)