Engine misses only after running awhile

Guest · Post by **Guest** » 27 Aug 2004, 09:03

86 740T, 161K. Weird problem. In the morning, car cranks on first turn, engine runs so smooth you hardly know it is on. However, over the last week the last couple of times that I've driven it kind of hard (extended driving @ 65+ mph), after I let it cool down and turn it off, when I restart it within 2 hrs or so it has to be cranked twice to turn over, and it runs with a noticeable miss, which varies from slight to so bad the engine has shuddered, vibrates hard and sometimes cut out. When this condition occurs techometer swings wildly between 2400 and down to 3000 where it almost shuts off, or does. I've done a super major tune up over the past two months (engine wires, distributor/cap/rotor/plugs). Fuel injectors/o-rings were also replaced). At first I felt like a vacuum line that might not be seated so that when things got hot and hectic under there air flow wasn't quite right, so I checked all of them and they all were secure and tight with no [obvious] leaks. Now I'm not quite sure where to proceed. Any thoughts?

LaRy · Post by **LaRy** » 27 Aug 2004, 11:13

The fact that the tachometer swings wildly and the engine still hardly runs tells us that you definitely have an ignition problem. The tacho is electrically connected straight to the ignition coil, input side, so if it is showing wild values, the ignition is working wild. Either it is the sensor or the power stage amplifier or bad connections. Most probably you have some bad conection between those two components causing this problem. Check also the connection at the ignition coil.

Volvord · Post by **Volvord** » 28 Aug 2004, 11:40

I agree, first start with the easy and inexpensive diagnosis. Clean and deoxidize all ignition connections and grounds. Secondly check that the gromet going under the distributor cap is not broken, if it is it can allow an intermittent short to ground that can cause very problematic symptoms. Third it could be a bad Hall Effects sensor, a bad Ignition Amplifier, or (as I found out) the 1985-1988 700 series used biodegradable insulation on the wiring harness that deteriorates. Take a look at the brown wires that ground the injectors on the intake manifold. If the insulation on these is badly deteriorated then there is a good chance that the wiring in the harness is also deteriorated and causing your problems.

guest · Post by **guest** » 28 Aug 2004, 12:09

I'm having the same problem, in my 88 740-GLE. It'll do fine until it will hit a hill, then it will seem to miss, and after while it will stall, even if i'm going 30mph. It happened 5 times in a 10 minute ride this afternoon... thanks for the answers!

Guest · Post by **Guest** » 30 Aug 2004, 06:50

Thank you for your recommendation. The wiring harness was changed two months ago, a [used] distributor and Hall sensor replaced a failed one. The weird thing is that it runs beautifully after having sat for awhile (three-four hours or such as overnite) until it has is fully warmed up and run for awhile, and then turned off, sit briefly, and THEN restarted. That is when I begin to notice the engine miss, even heard a slight valve knock, rough idling and noticeable loss of power, which only seems to be temporary, because as I continue to drive it a while in that condition, all of a sudden it regains it's power, smooth idle again until I shut it off AGAIN. I'm thinking if it is a connectivity issue [bad ground, faulty ignition connection, Hall sensor] it would be consistently rough idle or no start at all? I will go over this thing again though. I guess I should be thankful it's not raining much here.

Volvord 784VC wrote:I agree, first start with the easy and inexpensive diagnosis. Clean and deoxidize all ignition connections and grounds. Secondly check that the gromet going under the distributor cap is not broken, if it is it can allow an intermittent short to ground that can cause very problematic symptoms. Third it could be a bad Hall Effects sensor, a bad Ignition Amplifier, or (as I found out) the 1985-1988 700 series used biodegradable insulation on the wiring harness that deteriorates. Take a look at the brown wires that ground the injectors on the intake manifold. If the insulation on these is badly deteriorated then there is a good chance that the wiring in the harness is also deteriorated and causing your problems.

LaRy · Post by **LaRy** » 30 Aug 2004, 11:48

When I had a bad Hall-sensor, the engine sometimes stalled during running, but most often it did not start again. Sometimes it started after a while, sometimes next day. Sometimes it was working for a month. The Hall sensor is an electronic unit, and of cource the heat after driving could have affect to the function.
You said the tacho sometimes showed 3000rpm when running poorly. This means that you also have sparks when you should not have any. The engine will for that reason fire to early giving a knocking sound.
In my case when having the poor Hall sensor a noticed something very odd. When the engine was dead, I disconnected the plug for the Hall-sensor with ignition off, connected it again, and the engine started when I tried again. It worked almost every time. Try to explain that!

Guest · Post by **Guest** » 31 Aug 2004, 06:09

LaRy wrote:In my case when having the poor Hall sensor a noticed something very odd. When the engine was dead, I disconnected the plug for the Hall-sensor with ignition off, connected it again, and the engine started when I tried again. It worked almost every time. Try to explain that!

This one is a bit different in that the car cranks and engine fires up completely with each start, it's just that it runs with a miss, shudder, vibration, lack of power until it just clears up and starts to run fine again -- until I turn it off and restart it within an hour or so. Weird (at least for a newbie like me).

940 · Post by **940** » 27 Sep 2004, 08:06

Have you checked the coil's ouput voltage?
If it's too low, the engine will loose performance when getting warmed up.