1985 T16s - running rough, revs dropping, stalling

[James]

It appears to be time for my three-yearly visit here. This time, I have more symptoms than I know what to do with, but can't narrow it down to a single culprit. Any suggestions on diagnosis or what's most likely would be very welcome.

A few months ago, I had issues with stalling coming up to a particular junction - slowing from 30 (mph) to 20 and changing down to second. It happened once or twice, then I had a stallathon where slowing for any lights would trigger it. Swapped in a used but good turbo dump valve and a new line for it, and everything seemed ok. Then, it had a couple of stalls at the same spot, and I mentally made a note to look over the dump valve again.

Cut to a week or two later, and the Saab was on its first trip of over 5 miles for a while - on the way back, it started sounding slightly rough - just skipping the odd beat. 20-odd miles later, the idle revs were falling off, and it started stalling with the same dump valve-type behaviour. It was also running rough, though, so not exactly the same. Pushed on a little further, and it would stall at idle, and I had to keep the revs up to keep going. A short while later, the revs stopped staying stable at higher revs - rev it to 2000, keep constant throttle, and it would drop to 1000. At that point I pulled over somewhere safe - it wasn't going to make it home.

Since then, I've had the bonnet up, checked that the vacuum hoses were ok (cut back and refitted a couple of so-so ends), and that the turbo dump valve was holding pressure. Also changed the spark leads, one of which I think got snapped at the plug end by an enthusiastic recovery guy. Symptoms are - starts, runs rough then dies. If I put the revs up, it will rev, then suddenly change its mind (after a second or two) and drop to rougher running at lower revs. I'm also hearing a turbo-like howl I'm not used to. It could just be that I have the bonnet open and the door open, so I can hear the engine a lot better, but I thought I'd throw it in there.

It's also now reluctant to start a second time - i.e. I can fire it up in the morning, go through the cycle above, and then try to restart it 15 minutes later after poking around the engine bay, and it'll turn over, catch occasionally, but not start. No signs of fluid leaking anywhere.

Any thoughts?

[jskrebs]

Hall effect sensor in Distributor or Ignition Module. Also crack a fitting on the fuel rail as a quick check for fuel pressure.
On an 85 you must also suspect the wire harness and grounding points. Any weird things going on with the lights?

[James]

No weird things with the lights that I've noticed - just charged and reconnected the battery to rule out battery fade. Are you going for the hall effect sensor as a frequent culprit in ignition issues? I guess I've been assuming it's something in the vacuum-connected side of the ignition system due to the issues around braking. Just remembered that I once spotted the revs diving for the floor as I was braking (mostly had to keep an eye on the traffic), but it went well under the idle speed - somewhere under 200, I think. Clutch deployed, naturally!

[jskrebs]

I missed the braking aspect. Perhaps a failed servo? Would cause a large vacuum leak. Pull the vac line to the servo plug the manifold and try starting.

[James]

Checked the brake servo - no difference.

[jskrebs]

Check the vacuum advance capsule? Perhaps it sticks. I had one go bad in my NA

[peva]

You may have blown some rings. Do a compression test. Should be around 140psi. Check for mayonnaise in the oil and also listen at oil filler orifice, cap removed, with engine running. Check there is no puffing noise, indicating blowby, when you blip the throttle. Should be just a smooth muffled noise. Hope it's not this for your sake, but the symptoms are there.

[jskrebs]

Oh, No I hope not. Had that issue with mine. Was worse than blown rings though....

[James]

Sorry for the delay - first day I've been home in daylight - don't have the kit to do a compression test, but the oil is clean. Steering fluid is a little low, oddly, but I may have just lost track of topping it up. It's not running for long enough without a foot on the pedal to get a proper listen to the engine bay, unfortunately.

[James]

Update - I've struggled to find any vacuum leaks, and as it seemed to run smoothly for a second or two after starting, I was suspecting something in the engine management system. Pulled the air mass meter to get a part number off it, as it's the simplest part to swap out, then thought I'd try starting it - runs with a slight wave to the revs at constant throttle, but much better than it did before. So, sounds like the AMM at the moment.