B200F Engine Hopping on Idle

[KerPLoD]

I've looked at a few threads but haven't found anything so apologies if this has already been covered. If it helps, I have a 1992 B200F with Bosch K Jetronic fuel injection. It is serviced every year despite low mileage, and the spark plugs and HT leads were done in 2016 - it has since done about 2500 miles.

On idle, the engine shakes, making the car hop up and down slightly. So far, I've had a few things done;

-Running it on Shell VPower did noticeably improve things (although not by a huge amount)
-The intake manifold gasket was in pieces, so I replaced that so there shouldn't be any air leaks (made no difference)
-I also replaced both engine mounts with genuine Volvo items, and this improved things a little more, though barely noticeable.

So, does anyone here have any suggestions as to what I could try next?

Many thanks in advance,
Liam

[Nicholas Lewin]

Liam

Tell us more about the hopping as you put it...

All engine speeds? under load? when driving?

If it is only a vertical movement, I wonder whether your rear gearbox mount is in some way suspect?

[BrianH]

Look to the injector seals. There are two, one thin and large and one thick and small. The small one is most critical.

Easy check before this is spray something that will increase the RPM if there is an air leak

I hope its not corrosion of the petrol tank letting in water. That is not an easy solution. Mine was a breakers injection system. Worked well though and not expensive.

BrianH 245 B200E CI injection now LPG/petrol. 711000 miles from new.

[KerPLoD]

Originally Posted by Nicholas Lewin

Liam

Tell us more about the hopping as you put it...

All engine speeds? under load? when driving?

If it is only a vertical movement, I wonder whether your rear gearbox mount is in some way suspect?

When it idles, like I said. Neutral, at a standstill.

I suppose it's probably worth noting that when you first start the car cold, the revs fall and it almost stalls before the revs rise back up to idle revs.

[classicswede]

Originally Posted by KerPLoD

1992 B200F with Bosch K Jetronic fuel injection.

B200F would be LH2.4 and not K jet unless converted to K jet

K jet air leaks around the injectors are a common problem and first starting point

LH2.4, check for air leaks including the manifold gasket.
Clean throttle body and clean the idle control valve as your first steps

[AndyV7o]

As above, perished seals are likely, letting in extra air, also lets say cyl 1&2 still seal well, 3 is leaking and 4 pretty bad, youll get 2&1 firing properly, 3 less efficiently then 4 poorly, youll get a reciprocating rise and fall of revs in line with the idle speed.
When driving you are raising the rpm and varying it which cancels out the reciprocating action somewhat, you are also enriching the mixture, lowering the vacuum when the throttle is open, then taking in a volume of air much greater than that of idle, all of which can mask or lessen the problem.

As above, spray brake cleaner/carb cleaner/easy start around the injectors amd manifold area to see if it affects the engine. Any effect indicates it is being drawn in. Whilst you have worked on inlet, it could be cracked or warped also.

[KerPLoD]

Righto, thanks for the tips, could've sworn it was K jet, someone at one of the meets had told me it was. Now I know