steady misfire around 2K

[swisstoni]

My 95 4.0HC has a misfire that centres around 2K revs in all gears. At all other engine speeds it is silky smooth. In 5th at 2K, you can feel the power gently coming in and out. In lower gears it just feels jerky. I've has the car since new and it's only started this malarkey in the last 6 months. It's driving me potty.

I have had a few people look at it now and taken it to two rolling roads and nobody seems too sure what to do next. Some people say injectors other people say cam :-(

I'm not too keen to finance any more speculative investigations so:

1. anyone got any ideas - bear in mind I've had all the usual misfire stuff done already eg. leads and lamda sensors, plug shrouds..

2. Can anyone recommend a good diagnostics outfit in the Berks, Bucks, Surrey area ?

[HarryW]

At light loads and low revs, might be worth checking the vac advance on the dizzy is working.

H

[shpub]

Power Engineering, Uxbridge with Mark Adams.

[ribol]

I cannot see a Distributor advance problem causing power to come and go at a steady speed and if it has been on a rolling road twice I would have thought that would have been spotted. A worn Cam would cause a constant problem on whatever cylinder has the worse Cam wear and power would be down, should also get spotted on rolling road.

From your description the problem is constant at given revs; the injectors could well cause this. If you intend going down that road you could reduce your financial gamble by replacing them on one bank, test car and see if problem still exists. If problem still there put ones you took out in other side and see if that cures it.

Ivan

[trefor]

Could well be the injectors, or a dozen other things. They'll need cleaning at this mileage - I had to have 3 replaced ... not cheap at 30 quid odd per injector to clean ... then you pay for the new ones too ... not sure of the total price, but it smoothed my car out immensely. Mark then re-set everything up.

Interestingly, it had made 241hp on a previous Mark Adams sessions with flakey injectors (no time that day to do them) - they squirted fine when you squeeze hard enough - the slightly dodgy spray patterns and crud affected part throttle in my case.

[trefor]

ribol said:

I cannot see a Distributor advance problem causing power to come and go at a steady speed and if it has been on a rolling road twice I would have thought that would have been spotted. A worn Cam would cause a constant problem on whatever cylinder has the worse Cam wear and power would be down, should also get spotted on rolling road. From your description the problem is constant at given revs; the injectors could well cause this. If you intend going down that road you could reduce your financial gamble by replacing them on one bank, test car and see if problem still exists. If problem still there put ones you took out in other side and see if that cures it. Ivan

But don't play with the fuel system yourself unless you're knowledgeable on how to depressurise it etc.

[swisstoni]

thanks for the ideas guys - in fact I have done everything you have suggested (apart from start replacing injectors).

Mark Adams suggested Tower View as the next step to delve deeper into the mechanical side of things (who I'm happy to go to having used them before) but I just thought I might get lucky and find someone on here who fixed the same fault.

One last idea - would it make sense for a Fuel Injection specialist to have a look - perhaps they could confirm or deny a problem before I start swapping things out.

[david beer]

Try disconecting and plugging the vacuum advance pipe so it doesnt work.Cures mine, might be "masking" a problem but not noticed anything.