1991 740 engine misfire and rev counter erratic

[volvobusman]

Hello, we have a 1991 740 2.0 non turbo with a misfire on it and the rev counter is bouncing around too, the misfire is on acceleration but seems to go when in 4th gear around 30 - 40 mph, not tried anything yet as the car is 25 miles away at my father in laws, just thought I`d ask if any of you fine fellows have had this problem , cheers Andy, ps just been told the rev counter is working as normal at the mo and the car runs better but not 100%

[stephend]

If yours has the B200E engine (the one with K-jetronic injection) then it has a Hall sensor in the distributor. A bouncing rev counter and misfiring could be a problem with the sensor - or its wiring, which is a bit delicate and vulnerable where it passes out of the distributor housing.

[bob12]

It might also have the B200F, but whatever firstly I would take the plugs out and look at them and also look the plug leads themselves. I have had this problem in the past and you might well find that the leads are not making a proper connection (several reasons) on to the plug heads.

[Clan]

Originally Posted by stephend

If yours has the B200E engine (the one with K-jetronic injection) then it has a Hall sensor in the distributor. A bouncing rev counter and misfiring could be a problem with the sensor - or its wiring, which is a bit delicate and vulnerable where it passes out of the distributor housing.

It could also be the Ignition coil driver amplifier module or the "Supression" relay

the rpm gauge gets its pulses direct from the coil so it shows there is a poor triggering signal going to the coil , should be an easy fix once it is diagnosed . I have found the Hall device in the distributor suddenly fails rather than plays up , , but the wiring can get broken and be affected by the engine movement shorting the signal out .

However , a 1991 would have a B200f with a flywheel position sensor i would look at this first

[Prufrock]

Originally Posted by bob12

It might also have the B200F, but whatever firstly I would take the plugs out and look at them and also look the plug leads themselves. I have had this problem in the past and you might well find that the leads are not making a proper connection (several reasons) on to the plug heads.

I agree with Bob, you should check the leads, and that they aren't silicone (as I've mentioned), they can be unreliable (even when new), copper wire leads are preferable.

Jon.
700/900/90 Register Keeper VOC.

[Clan]

Originally Posted by Prufrock

I agree with Bob, you should check the leads, and that they aren't silicone (as I've mentioned), they can be unreliable (even when new), copper wire leads are preferable.

Jon.
700/900/90 Register Keeper VOC.

The fact that the Rev Counter is being affected means the coil is not being triggered properly as the rpm signals are taken direct off the coil -ve terminal via the Red/White wire , the coil is downstream of the HT leads so it will not be those .. ( causing this particular problem anyway )

[Prufrock]

Originally Posted by Clan

The fact that the Rev Counter is being affected means the coil is not being triggered properly as the rpm signals are taken direct off the coil -ve terminal via the Red/White wire , the coil is downstream of the HT leads so it will not be those .. ( causing this particular problem anyway )

I was not suggesting the HT leads were the cause of this particular issue, I commented that the OP had changed them and was asking a simple question (initially) - were the replacements copper wire or silicon (silicon leads can cause problems) ?

Copper wired leads should be the preferred choice.

Jon.
700/900/90 Register Keeper VOC.

[Clan]

Originally Posted by Prufrock

I was not suggesting the HT leads were the cause of this particular issue, I commented that the OP had changed them and was asking a simple question (initially) - were the replacements copper wire or silicon (silicon leads can cause problems) ?

Copper wired leads should be the preferred choice.

Jon.
700/900/90 Register Keeper VOC.

Hi Jon ,
Yes the wire leads used on the 240 last forever really , the later graphite/carbon core ones on the 900 can cause problems especialy the coil HT lead .

[paddingtonbear]

I had a similar problem during the last saga but one and it turned out to be loose connections on the alternator.

[volvobusman]

Thank you to those folk that took the time to help out with my problem, hopefully I will be looking at the car on Wednesday, let you know what I find, cheers Andy