1985 745t

Recently whenever I first start driving my car after it has sat and gone cold the engine is incredibly retarded. It will run alright for most of the black in the turbo gauge, but the second it approaches the yellow boost area I can feel it turn into a slug (like its struggling for air or being held back).

I had this kind of problem more erratically before (it woiuld "suddenly" get retarded when it was hot out in the summer on occasion) and solved it by reseating the AMM connector at the AMM (unplug and replug). I haven't tried that now, but it seems like its currently to regular (only when cold) to be solved that way. Additionally the fact that it goes away on its own when the car warms up leads me to believe it isnt the connector.

So does anyone know what this might be? Is my AMM dying? Something else?

Thanks,
Rick

I'm going to take a wild stab at this.

Per chance, do you have the airbox preheat inlet closed off and the thermal valve tied off? Is the preheat side blocked off with a pipe 'cap?' If so, is it possible that whatever you tied it off with has come loose and as a result, when your car is cold, it's trying to draw in air from the side that's blocked off?
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Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'

Thanks for the response. No, I never bothered doing the fresh air only inlet block off of the "pre-heated air" intake. I figured the cars pretty old, so if both parts haven't failed yet they are due anyway and didn't want to mess with the airbox.

If the airbox temperature rod was bad, however, I believe it dies in the "warmed air" mode? Hence the common problem of bad temp. rod killing AMMs, but if that were the case wouldn't I see my problem when the car is warm and not when it's cold? o I would expect the opposite of what I'm seeing if the airbox temp. switch were bad.

It also isn't temp sender since I replaced that not too long ago. Just kind of boggled by what this could be... could excess blow-by with highly worn rings cause it? When cold the rings would have the greatest gap from the cylinder walls, but I wouldn't think I would see such SEVER retardation.

Today was warmer and the car was sitting in the sun and the problem disappeared. It thus seems to SPECIFICALLY be linked to ambient and physical temperatures...

So annoying tracking these intermittent sensor problems.

Thanks,
Rick

My 85 745t is also having a bit of the same problem, but not that severe. I have to back off of the gas pedal sometimes when the engine is cold. I don't really know what the problem is, but I have done a lot to my car so this might lend some insite to the problem.

Mine car has had all of these things replaced:

airbox themostat, all temp sensors, main fuel pump, fuel pump relay, all battery cables, battery, fuel pressure regulator, idle air valve, most vacuum hoses, spark plugs, ignition wiring, distributor, igition coil, and a bunch of other things not related to this problem.

What I have not changed is the AMM and the O2 sensor. I set up the AMM idle adjusting LED and had to turn the AMM adjustment screw quite a bit for the LED to cycle properly. I am thinking that the problem is one or the other or both. I don't know the age of the O2 sensor, but from reading the FAQs it leads me to believe that it would not hurt to change the O2 sensor. The AMM is a bit more expensive, so I am in no hurry to change it. I am a bit worried that the AMM may not be working up to par because my airbox thermostat was broken and stuck in the open position before I fixed it. I do know that the contacts on the AMM connector get quite oxidized on the bottom side of the pins. This oxidation is black and non-conductive.(it is actually hard to see) You should be able to wiggle the AMM connector while the engine is at idle without the rpm changing. If the engine RPM varies when you wiggle it, you need to get those pins cleaned up.

I will let you know if the O2 sensor fixes mine when I get around to it. I am trying to make my car as reliable as I can since I have to drive it 60 miles to work every day.

Randy

Interesting. Well, you've basically covered all the alternative items I thought might also be responsible. I had beeni thinking that it might have been a coil, spark, etc. issue, but if you've replaced most of the ignition stuff I guess that's pretty much ruled out.

So it woudl seem it is something related to either the o2 sensor or the AMM. I'm leaning towards the latter given the fact that messing with the harness cured a very similar problem previously, and I'll be checking for oxidation on the underside of those connectors in the near future.

One other thought I had, a bit more ominous, involved a very slow coolant leak in the head gasket. Essentially I was thinking that maybe coolant leaks a little bit into the cylinders after the car sits overnight if its cold (since the head would contract slightly when it is colder). If that's the case maybe its enough to adversly affect the o2 sensor's reading until the trace coolant is burned off and the engine reaches operating temperature.

But wait, isn't the o2 sensor bypassed when the car is first turned on until it reaches a certain operating temperature? Like it doesn't actually serve as an input when teh car is cold but is bypassed by the ECU in terms of mixture?

ARGH... such a PITA. Thanks for the post, I'll look forward to seeing a post on your experience after you r&r the o2 sensor. Interesting that we're having the problem on the same vintage... mine is a 1985 as well.

Merry Christmas.
-rt

Yeah, that's what happens when it dies usually, it gets stuck in the hot air mode. But what I was hoping was the case was that you had the hot side sealed off and the working valve was pre-emptively tied over like some people suggest.

In that case, if the knot broke, the valve would spring back to the warm air side, which was sealed off and would explain why when it warmed up it went back over to cold.

But that's obviously not the case.

Hmm...
--
Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'