Surging idle only when warm...

Hey folks,

I have recently returned from Glastonbury with the Classic ('89 3.5 efi, 41k miles) and at the start of the journey, the old girl was playing up as soon as she warmed up: the idle was all over the place and I was having to left foot brake to stop her from stalling. This lasted for about 15 minutes in the traffic coming out of Glastonbury and almost as if a switch had been flicked, it stopped and returned to a regular, smooth idle. For the remaining 400 miles back to Glasgow, she didn't skip a beat, with regular stops and restarts posing no problem at all.

I've no idea what the issue is; I wondered if it may be the idle control valve getting blocked but I'm struggling to find it on the plenum, it doesn't seem to be where others have described it to be. It also makes little sense why it would do it when warm then after running for 15 mins at warm, return to normal again. The sparks are all new (less that 1 month old), as are the leads and oil and oil filter. The old girl even managed 24mpg with 4 up and a boot full of camping gear, which was far more than I anticipated but I'd like to get this sorted as it's not ideal.

Any help would be appreciated guys. Thanks.


(Just arrived here having done 430 miles...)

HI.

There is no idle control valve - that came with the 3.9

My 1988 3.5 efi used to do the same as yours and i eventually cleared it after a good clean round the air intake butterfly, the rocker cover breather, and replacing the vacuum advance unit. I couldn't tell you which of those things actually fixed the problem though!

Hopefully it won't happen to you again now.

Good luck!

Pete

Thanks a lot for your help Pete, that's interesting to know there's no IAC valve and also explains why I can't find it

I'm going to tackle it on Tuesday; I'll take the plenum off and have a good clean around...I'll report back with pictures and updates.

Thanks again!

report back when if you have success

Updates:

First start in exactly 2 weeks, started on the button, idled fine for 10 minutes then back to the irratic lumpy idle as described above, albeit not as violent as before and no stalling.

I'm going to strip the top tomorrow and take the throttle body off; in the meantime, any ideas as to what the following components are (circled in red):

...ending here...

And i'm a bit concerned about the circled red bit below:


Should this have a hose attached to it??

Any help is, as ever, appreciated greatly. Cheers guys.

First is yer flame trap. Take it off and clean it out with petrol or meths and then let it dry. Ideally take the gauze out to do it.

Second is yer crank case vent recirculation pipe. Take it off if yopu want and make sure it's clear.

Thanks a lot, I appreciate that.

Is the third the vacuum advance? If so, what does it do?

I think it provides ignition when the engine is not pulling a lot or is just cruising along or coming to a halt. Also used when engine is cold or a lean mixture, I think. I'm not a dizzy expert.

oh, and check all vacuum hoses for holes!

PS is the engine just surging or is the RPM fluttering as well?

Fuel filter?

Can you borrow an ECU from someone? I had the same issue and it was a faulty ECU.

thomsonaj said:

Thanks a lot, I appreciate that.

Is the third the vacuum advance? If so, what does it do?

Click to expand...

Third is the vac advance unit - in relatively simple terms it advances the ignition timing under acceleration using vacuum caused by opening the throttle, it is not the same as the inlet vacuum though because the vac advance pipe is connected to the plenum just behind the throttle butterfly and is ported so the vacuum is only created when the throttle moves (opening) rather than being subjected to constant vacuum (negative manifold pressure) but when the vac advance unit packs up, the diaphragm inside it usually splits and causes a vacuum leak which in turn tends to cause idle issues because it is the same as having an air leak on the inlet which affects the fuel mixture.

Thank you all very much for your responses up to now: I stripped down the vacuum hoses and had a cursory look at them all; no sign of any holes but the rubber looked a bit perished around the joins so I clipped them and resealed.

The upshot is, she's idling ok so far, no nonsense when i was running her. It was definitely idling more smoothly and steadily so whatever I've done, it's helped something.

On the downside, I went to clean the plenum out and snapped my hex bold head for my wrench trying to get the bolts off...so next purchase is a new heavy duty bolt set to try and get that off...joy.

I'm still a bit concerned about the "missing" pipe on the vac advance unit; can this be taken off the dizzy and inspected? I'm a bit wary of fettering with the dizzy and screwing the timing up...

Progress so far folks, and I'm grateful of all your help so far. Thanks.

thomsonaj said:

I'm still a bit concerned about the "missing" pipe on the vac advance unit.

Click to expand...

Don't think you have a 'missing' pipe. Mine's the same (1989 3.5 Efi). Two sort of metal pipes coming from the vac advance on the distributor, one of which goes into a vacuum pipe and up to the plenum, the other just goes to nothing, like yours.

Good luck with it.

Have you downloaded the flapper diagnostics manual from lr4x4.com tech archive? Handy to work through and give the system the once over.

The grey thing on the top of the rocker cover is the engine breather flame trap. At the other end of the pipe is where the oily mist that goes through the flame trap is drawn back into the engine & burnt. Inside the plenum casting is a gallery along which this mist passes. This gallery can block - mine was when I bought it - and the result is crankcase pressurising & oil leaks from all sorts of interesting places on the engine. Mine was so bad I had to pull the metal 'in' pipe out of the plenum & use a twist drill (finger power only!!) to break up the crap.The other end of the gallery opens into the plenum on the engine side of the butterfly.
A build up of crap in the plenum intake around the butterfly can also effectively fully close the butterfly - it needs a small gap which is set by feeler gauge.
Ignore the short metal pipe on the rear of the vacuum advance capsule. Nothing goes on it. It should be open to atmosphere. Suck on the pipe which goes into the front of the capsule ( the rubber pipe that comes from the top of the plenum), with the engine running this should slightly increase the revs. If it doesn't - or there is no resistance to the suck then the vacuum unit is dead. DO NOT poke anything into the unit. You WILL damage the diaphragm & have to buy a new capsule. (Guess how I know)
Air leaks on the intake side will upset the idle.

I have had a couple of ECU's go faulty. One ran like a bag of spanners & the other went VERY rich. Confirmed with a known good unit which restored normal service. They did not manifest as an intermittent fault. Flapper 3.5 EFi system.
Common advice is to change the coolant temp sender - the one behind the easy one - as a matter of course before doing anything else. I've had a temp sensor fail on another vehicle & it was initially an intermittent fault which periodically caused the vehicle to stall at idle (junctions etc) followed eventually by complete failure which prevented the car starting at all.

If you don't slacken the dizzy clamp - allowing the complete dizzy to move - you won't affect the timing. Its the 'Y' shaped thing held by a bolt that goes into the engine block near the point where the bottom of the dizzy disappears
into the block.
Have a quick read through this before doing things to the dizzy, esp the warning about pulling the rotor arm off, the problem he had wasn't too much fuel!: http://forums.lr4x4.com/index.php?s...189315&hl=much fuel&fromsearch=1&#entry189315

ETA What is it with this website that causes phrase like 'engine block' to link to other websites?