The car: 1990 740 GL, Rex/Regina. Runs poorly on startup for about 2 minutes then runs great.

I just checked the air temperature sensor (just downstream of the air box). It's reading about 1600-1800 ohms while the ambient temperature is about 60. The engine is a little warm though... but I'm guessing the actual sensor can't be experiencing more than 70, 75 degrees tops.

Could this thing be causing my issues? If so, what's the cheapest source?

Thanks

Well I think I found the problem. Egg on my face.

The car had sat for a couple of days so I started it and pulled each plug wire in turn to see if one cylinder wasn't getting spark during the 'rough idle' period. All cylinders were getting spark.

So I decided to pull out the plugs so I could use their condition to help diagnose the continuing problem. Hoped to find one cylinder with evidence of too much 'lean', to find a possible bad injector. These are new plugs I put in last January.

Turns out 2 of the plugs were loose. While I had torqued them to spec, I guess the block was the wrong temp when I put them in, and they had loosened more since then. I mean REAL loose, like 2 turns loose. They must have vibrated and backed out slowly over time, which would account for why the condition was getting worse over time.

Of course, with 2 plugs backed out that far, the spark wouldn't be in the right spot in the combustion chamber, AND it would mess up the compression on those 2 cylinders. And once the engine had warmed up some, the cylinder would fire better even with the spark in the wrong place, and the head's expansion would tend to seal the thing up so the compression would improve. makes sense, right?

Anyways I put the plugs back in to the proper torque spec while the block was about 70 degrees F. And the car started right up without hesitation, and it started again this morning without issue. crank crank VROOM.

Bottom line: if you have an issue with starting/running that goes away after a couple of minutes, check your spark plug tightness.

cheers

Thaddeus

This does not resolve your cold start issue, does it?

It may! I'll have to wait and see.

I side with Texan745 on this one. A vacuum leak could make the mixture too lean until the O2 sensor comes online (in about a minute) and reports the lean mixture, then the ECU corrects for it.

In 5+ years of owning a Rex/Regina car—resulting in almost daily scans of this forum—I have never experienced (or seen reported) any confirmed Air temp Sensor or ECU problems.

A bad air temp sensor would set a code and the CEL/MIL light. The "running better" result is probably subjective anticipation after spending money.

IMO, it's the normal age/wear things more often than the electronics.
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Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.

I have an 89&just found a leak where the idlevalve hose was rubbing the air cleaner housing.&or you may have an air leak around the throttle body ,like the o ring that separates the tps from the throttle body,,or around the throttle itself.

vacuum leak.

Replaced it. Did not fix problem. Car runs better though.

With electronics, you find an error condition, you correct it, whether it fixes the acute problem or no. Pays in the long run.

I'm running out of stuff to replace, though. I'm wondering if the ECU is jacked up. This is kind of acting like a cold solder joint that closes once it warms a little and starts working. I dunno.

Sigh.

You may be right about the ECU. My 1989 has had the same cold rough idle for about one minute ever since I swapped my pink label #561 ECU for a #951 which is supposed to have a better reputation than the 561.
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'89 silver 244DL in Milwaukee WI

Here's a link. They sell it for about $55. I don't know if you want to invest in what may not fix your problem.

http://www.google.com/products?hl=en&q=air+temperature+sensor+volvo+740&um=1&ie=UTF-8&checkout=1&sa=X&oi=product_result&resnum=11&ct=checkout-restrict