For the last month or so, my 2-litre Volvo 480 (B20F engine) has been plagued with an intermittent stumble when it's idling. It doesn't always do it though, and I can't always make it do it, even if I want it to. A typical example would be crawling along in slow moving traffic, say revving up to
2,000RPM in 1st/2nd gear whilst setting off, and regularly stopping with the traffic. When I come to a complete stop, I can feel it missing ever so slightly. If I set off again, even only moving forwards 10 metres, it may do it again when I stop next, or it may not. When it doesn't do it, it idles perfectly!When it misses, I fan feel it do so, the rev counter needle 'blips' and the oil pressure guauge blips too. It only ever misses when idling at
800-850RPM. If I push the accelerator slightly and increase the revs to 1000RPM, it idles fine. The idle speed is controlled by the ECU, so I'm not able to mask the problem by simply increasing it (not that I'd want to mask it - I want to cure it!) When it's stumbling, it's fairly regular/rythmical, so I assume it's just one cylinder, or an item related to one cylinder that's causing it.Other than that, it seems to run almost perfectly. There is constant power throughout the rev range, with the exception that there it stumbles momentarily if I floor the accelerator from idle. All of the things I've tried (below) haven't made any improvement at all.
Things I've done to try and fix it:
- Cleaned the internals of the idle valve. Disconnecting this (electrically) makes the revs increase to about 2,000RPM at idle, and reconnecting it brings them back down to 800RPM, so I'm fairly confident it's working fine.
- Replaced spark plugs, HT leads, distributor cap, rotor arm and air-filter.
- Cleaned the throttle body with carb cleaner.
- Cleaned the air temperature sensor (in air-filter to throttle body pipe) with carb cleaner.
- Checked all vacuum connections, and used very tight cable-ties to make sure they are sealed properly. Replaced several sections of vacuum tube that looked worn.
- Replaced a split hose from vacuum storage tank.
- Cleaned the flywheel sensor. This looked quite dirty, and was rusty on the bottom of the sensor itself, so I removed the rust with some very fine wet'n'dry. It didn't make a difference though. Exactly the same as before.
- Put some Redex injector cleaner in the fuel tank. I don't think this will help much, but I thought i'd give it a go anyway!
Things I've yet to try:
- Have bought a new fuel filter to fit at the weekend. I don't think this will solve it though.
- I've bought a vacuum pressure gaguge, hoping this will tell me if I've got a sticking valve. This is fairly high on my list of suspicions now. Would this sound feasable to only cause a problem at low revs, and be intermittent?
- Testing the injectors. I've no idea how to go about this. Anyone know?
- Checked for gasket leaks around the manifold. Again, I'm hoping this will show up when I try out the pressure gague.
Has anyone got any other suggestions of things it could be? I'm open to all ideas, as it's really starting to bug me!
Any help, greatly appreciated!
Ben.