740T bucking when cold

Distributor leads? That's what caused similar probs with mine, specially when it rained.

I can check them, they're Magnecore wires though and only a couple years old. Hasn't rained lately either. I haven't checked the distributor cap lately though.

That is odd. How about the operation of the throttle position switch? Maybe it's getting balky when cold and isn't coming out of idle mode. Listening for the click with the engine off is a good start.

But it always could be the AMM/connector.

Mike

I hadn't thought of that switch, I think I've eliminated it though as the last time I had this problem I was coasting with the clutch in and the engine was idling very rough and stumbling still, just as it does with more throttle applied, it doesn't seem to make a difference.

Ugh - I'm no expert on that but it does make me think of the AMM. The connector on mine acted up, causing similar symptoms, and with it barely idling I pressed on the connector a bit and the idle jumped up (like starting) and then settled down. Cleaning the connector with alcohol (because that was the best contact cleaner I had around) fixed it right up.

Mike

That had been my original suspicion, I'd just wanted to check if there was anything else likely. I'll try cleaning the connector and see if that helps, if it starts having the problem more regularly it'd be easier to track it down. I do need to figure it out though, it's dangerous, I'm afraid I'll forget and charge out into traffic and end up broadsided when I lose power.

This is undoubtedly happening just as the engine computer systems are going closed-loop. You undoutedly have one or two timed vacuum switches screwed into the block and/or cylinder head. When the coolant is warming up, these thermostatically timed vacuum switches open to allow vacuum to various other components, one of them being the charcoal canister, turbo overboost switch and in the case of my old '84 Turbo, the distributor advance. It might be time to check ALL of your vacuum lines and replace them. I just replaced all of my lines...took about two weeks and made a HUGE difference. You should have seen the condition of some of them, they were all sucking air. My old Volvo, from the day I drove it from the dealer has had that idle decrease and a bit of rough running during warm-up, particularly on very cold mornings just before the engine goes closed-loop.

James Sweet wrote:

Have we determined whether or not the OP has a turbo or not? I know that not all Volvo's with a "T" in the designation have been turbos in the past. I don't think that any Volvo turbos had an MAF sensor.

Michael Pardee wrote:

Mine does :-) / :-(

Mike

It can't be going closed loop *that* quickly, it does warm up fast, but the first time it did it the engine had been running literally no more than 10 seconds and it was below freezing outside. I'd hopped in, put on my belt, started it up, put it in gear and started to back out of the driveway when it started lurching like crazy. The second time it'd been running for maybe 30-40 seconds.

Also I'm pretty sure it doesn't have any of that vacuum stuff, my 240T has that, but the 740 is LH-Jet EFI.

Yes it's a 740 Turbo with a B230FT, hence the T I put in the post, the only Volvos with a T in the designation and no turbo were the GLTs. Only the 240 Turbos had K-jet, the 700 and 900 series all had either LH with the AMM or possibly Regina, not sure whether the turbos got that or not.

May be time for a new cap and rotor?

Sounds like a problem with the fuel enrichment, possibly the thermo-time switch, injector, or wiring.

There is no thermo time switch that I'm aware of, wiring is a possibility, the original harness is a bit tattered, I suppose I should replace that too.

Have you tried cleaning out the slops in the air fuel mixture gizmo under the intake manifold .Try cleaning its electrical connection as well .It gets messy when you use normal oil and needs a good flush out so it can slide in and out to alter the mixture .Its easy to remove and clean .Now your heading for winter ,it might be more sluggish with thicker glug .

Maybe not. I'm only going by the generic L-Jetronic description, which describes a cold-start valve and a thermo-time switch. These add fuel under the conditions you describe. Or your auxiliary air device, which adds extra air during cold-start conditions, is intermittent.

Why replace the harness? You can check the wiring using a VOM and judicious bending and tugging.

I'm not aware of any of that stuff, there's a temperature sensor, I'm not sure if that could cause this problem though? It has a CIS valve to regulate the idle by bypassing the throttle plate, that part seems to be working.

Because it's clearly rotting out, there's insulation falling off exposed wires all over the place. I've taped most of them up and haven't had any trouble with them yet in the 5 years or so I've been driving it, but it definitly needs to be replaced. Classic pre-89 Volvo problem.

Air fuel mixture? You mean the idle speed valve? That has nothing to do with mixture, it allows a variable amount of air to bypass the throttle plate to regulate the idle speed. I cleaned it out about a year ago, might be time for it again but I really don't think that's my problem here since I've experienced it both at idle and all the way up through WOT. Once it happens though it seems to not happen again until the engine is very cold.

"It has a CIS valve to regulate the idle by bypassing the throttle plate, that part seems to be working."

That's the aux air device. It would open fully during cold cranking and gradually close as the engine warmed up, finally remaining open just enough to hold a steady idle. Some people clean them out with carb cleaner when they start to stick.