M5 Guru (>2000 posts)
Join Date: Jul 2012
Location: Jakarta, Indonesia
Thanked 202 Times in 184 Posts
Unusual rough running problem
I have a short 4 km drive to the office each day.
This morning while closing the gate I noticed the idle rpm surging from 600 to 1000rpm , I turned the engine off and restarted and it was back to normal at 950rpm.
About 1km from home I was getting a hestitation on normal acceleration up to 2,500rpm , if I backed off the throttle slightly it would clear up and I continued on my way.
About 2.5km into the trip the engine rpm dropped to 400rpm at idle and then the engine cut out, it restarted on the 2nd attempt and I drove about another 300 metres before the engine stopped again this time accompanied by the smell of raw fuel.
The next attempt to restart resulted in a 3 ft long flame coming out of the exhaust (according to a startled observer) and the backfire blew 4 of the 6 ITB rubber caps off the ITB ports.
I managed to find 2 of the 4 missing caps , put them back on and tried again , the caps flew about 10 metres landing on the other side of the 2 lane road.
The next few start attempts the engine didn't want to fire at all so I decided to let the M5 sit for 10 mins while I arranged for a tow from my office with the office delivery truck.
I decided to give it one last try before towing and the engine kicked and died , 3 more start attempts and it slowly came up to normal idle speed. I then drove the M5 to the office, the hestitation was there but getting better and now smell of raw fuel.
After arriving at the office I had a meeting for an hour and I went out and started the M5 and the start and idle was totally normal.
I then disconnected the battery for 20 mins and it started straight up with a smooth idle.
What would be causing this backfire ?
To me it looks like the computer is either advancing or retarding the ignition to a point when the engine won't run.
DME fault or bad connection ? Intermittent fault with the crank sensor ? TPS ?
Anyone seen the HAL-like (2001) behaviour before ?
Tony, Aussie expat in Jakarta Indonesia
1990 E34 M5 S38B36 GA25324