1990 740gle drivability problems

djzuelke · Post by **djzuelke** » 08 Oct 2009, 13:50

After putting the head back together after a broken belt, I am having difficulty diagnosing an unusual problem. Idles normally and runs well at 2500 + rpms. When the accelerator is depressed engine revs to about 1500 or so rpms and then starts to lope. Timing light reveals wild fluctuations in timing at mid range speed. Also, the tach shows sudden drop without a corresponding drop in rpms. When accelerator is further depressed the engine suddenly revs and the tach jumps up accordingly. I have swapped out several components including the throttle switch, crank and MAF sensor as well as the ECU with no change. I suspect the ignition system control unit, located on the driver's side under the the dash in my GLE, may have an electronic fault. Any thoughts on this? Also, does anyone know whether or not this unit is interchangeable with other 740s?

Post by **billofdurham** » 08 Oct 2009, 15:29

Before throwing lots of money at the car check the crankshaft pulley. On the 740 this is a harmonic balancer made up of three parts. The inner part is bolted to the crankshaft. The center part is a rubber ring. The outer part is the one with the timing marks and pulley cuts for the belts. When it is new all of these parts are attached. As it ages, the rubber part becomes dry, the two metal parts are then able to move around this rubber part. The timing marks then move around the inner part. The timing is not affected as the inner part is keyed onto the crankshaft. However, if you start to time the motor the normal marks are chasing each other around the crank and make no sense at all.

For a better understanding of this problem run a search using author Kmaniac in California USA. This is a good write up of the things tried, and failed, which were resolved by replacing the crankshaft pulley.

Bill.

djzuelke · Post by **djzuelke** » 09 Oct 2009, 08:33

I checked out the pulley when I replaced the belt by referencing the crankshaft key as well as No. 1 piston at TDC. Besides, if timing was way off I doubt if the engine would idle normally in addition to running well at higher rpms. It is just at that rather narrow range of between 1500 to 2500 when the engine "lopes" and the tach suddenly drops. When accelerator is depressed to passing gear, it has plenty of power.

Post by **billofdurham** » 09 Oct 2009, 12:25

The inner part of the harmonic balancer is keyed to the crankshaft. However, the outer part is attached to the inner part by the rubber and is not keyed to the crankshaft. The outer part has the timing marks on it and they will chase all over the place if the rubber is worn.

As I said in my previous post the timing won't be affected but if you try to re-time it the floating outer section makes it nigh on impossible as the timing marks are doing their own thing.

This is a common fault on these cars as they get older.

Bill.

djzuelke · Post by **djzuelke** » 09 Oct 2009, 20:55

Bill, thank you very much for responding to my inquiry. Alas, my problem is not as simple as a faulty crank pulley. I wish it was! That was one of the first things I checked. Furthermore, the timing was never changed. As I mentioned before, the engine runs very well at idle and up to about 1600 rpms and then again above 2500 rpms. What could cause the timing to fluctuate only at the 2000 rpms plus or minus a couple hundred? At the same time what would cause the tach needle to suddenly drop as though its circuit was subjected to a load only during that stage?

djzuelke · Post by **djzuelke** » 20 Oct 2009, 11:56

Thought I would try one more time: What would cause uneven running and wide variation in timing accompanied by a sudden drop of the tach needle - but only in the 1700 - 2200 rpm range? Hot or cold, it runs good above and below the aforementioned speed. Clearly the uneven running (surging/loping) is caused by the timing fluctuation but what could cause this? And what would cause the tach needle to drop at the same time? It jumps right up and reads accurately once out of this range. Help!

Post by **billofdurham** » 20 Oct 2009, 12:05

I have merged the two threads for continuity.

As we hadn't heard from you I thought you had fixed it.

One thing I didn't ask - are any fault codes stored?

Bill.

djzuelke · Post by **djzuelke** » 21 Oct 2009, 06:50

No codes Bill. Today I will double check grounds and swap out the knock sensor. Any other suggestions?

Post by **billofdurham** » 21 Oct 2009, 12:19

With no codes we go back to basics.

When you replaced the head did you replace the exhaust and inlet manifold gaskets?

Other things that come to mind are:
The engine speed sensor which is mounted on the bell housing at the rear of the engine. The lead can be accidentally disturbed while you are working on the head.
Throttle switch and connector.
Check all the sensor connections.

I suspect the ignition system control unit, located on the driver's side under the the dash in my GLE, may have an electronic fault. Any thoughts on this? Also, does anyone know whether or not this unit is interchangeable with other 740s?

This was in your first post and I overlooked it. Some of the control units are interchangeable but it is a minefield as there were so many variations - EGR; non EGR; chassis numbers; engine type etc.

Keep us informed.

Bill.

djzuelke · Post by **djzuelke** » 30 Oct 2009, 08:08

Replaced the knock sensor, cleaned all the grounds; still the same problem - and yes, intake and exhaust gaskets were replaced. Any other suggestions? Anyone know where I can get an ignition control unit that will not cost more than the car is worth?