740 Volvo IAC issue

Bill Technical Support Specialist Mesa, Arizona Posted   Latest   Edited  
Question
Driveability
1988 Volvo 740 GLE 2.3L (8 B230F) 5-spd (M46)
Extreme High Idle

Looking for a sample waveform and some input on idle issues. From memory, the usual cause if throttle valve switch (not a TPS). Service info was not much help.

Throttle is clean and the hard stop and idle switch are adjusted. IAC was replaced with a new spare I had in stock that was destined for and never used on another project.

It has been so long since I have messed with one of these and coupled with it being my daily driver and the middle of the summe, I am nearly at wits end.

Beevo

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Ray Diagnostician
North York, Ontario
Ray Default
 

What year is it? Engine size?

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Tommy Owner/Technician
Houston, Texas
Tommy Default
 

Im assuming there is no vacuum leaks?

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Bill Technical Support Specialist
Mesa, Arizona
Bill Default
 

Correct. If the large hose to the IAC is pinched off, the idle is normal, well as normal as it will get based on min throttle plate and air bypass.

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Bill Technical Support Specialist
Mesa, Arizona
Bill Default
 

Well I filled out the vehicle information when I made the post, guess that does not make it with the posting. {:o) '88 740 2.3L turbo manual trans, wagon

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Jeffrey Manager
Yuma, Arizona
Jeffrey Default
 

I see there are 2 control circuits to the ecm, is this valve a stepper motor?

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Bill Technical Support Specialist
Mesa, Arizona
Bill Default
 

This would be my assessment. Service info is non-existant on the valve. Same SI also shows a thermotime switch in the sensor/control/switch pick list... The valve reacts if I close the throttle switch, either by letting up on the foot feed or disconnecting the harness and jumping the two terminals. It will pull speed down close to where it should be and then rocket back up. Looking more…

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Ben Mechanic
Chesnee, South Carolina
Ben Default
 

Doesn't this one have the adjuster screw at the MAF to tweak? I can't recall the OEM spec resistance they called for back in those days but I sure remember drilling small holes in the plug and tweaking the screw when it checked out of spec. As I recall it directly affected idle mixtures in open loop and indirectly otherwise.

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Bill Technical Support Specialist
Mesa, Arizona
Bill Default
 

The adjustment on the MAF is for mixture. There is no resistance spec, I think you might be thinking of the vane air flow meter Not a mixture issue I am chasing, 2800rpm at idle is the issue.

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Ben Mechanic
Chesnee, South Carolina
Ben Default
 

There is indeed a resistance spec for the hot wire MAF feeding B230FT's. I still have some of the tech training stuff socked away from when I was at the dealer in the late '80's. With the MAF unplugged the resistance between terminals 2 and 3 should be 3.5 and 4.0 ohms. As I recall we tweaked a lot of them at their 1200 mile break in service but not too often after. The majority never had the…

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Bill Technical Support Specialist
Mesa, Arizona
Bill Default
 

I doubt this is where I need to be headed for a 2500 rpm idle. I always set the "screw" with a 5 gas and the O2 unplugged.

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Jeffrey Manager
Yuma, Arizona
Jeffrey Default
 

Was gonna ask about that but since the car runs it's not high on my list. Should be a quick thing to check though.

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Bill Technical Support Specialist
Mesa, Arizona
Bill Default
 

Not high on mine as the power feed comes from a splice that feeds the ignition module among other things. I would have been walking for a while at this point,

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Adam Technician
Maplewood, Minnesota
Adam Default
 

Its been a while...so sorry if i get any specifics wrong. Its 3 wire, power on one and duty cycled to ground 50/50, 80/20, 70/30, etc. depending on what the computer wants for idle. As long as the idle switch is showing closed @ idle, i would maybe check the ECT sensor next. It will be under the intake, between runners 3 and 4. The ECT sensor connector and knock sensor are identical on some…

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Bill Technical Support Specialist
Mesa, Arizona
Bill Default
 

Insulation shredding, ah yes.... Mostly yellow and green wires on 240's and SAAB 900's of the same vintage. I removed and rewired an entire under hood harness on my '85 twincam turbo, fun weekend and my wife got pretty good at helping to re-wrap harnesses. {;o) I am very aware of the ECT and how darned fun it is to get to. I will do a temp/resistance test later this week. Fuel economy is off…

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Adam Technician
Maplewood, Minnesota
Adam Default
 

sure sounds like you have an open circuit in the ECT somewhere! for testing, due to that great location of the ECT, I prefer to get at the fuel computer, pull off the cover and backprobe for voltage....been burned by resistance checks too many times.

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Bill Technical Support Specialist
Mesa, Arizona
Bill Default
 

Actually it is not terrible to get to, remember I worked on SAAB's so double jointed appendages are standard equipment. You have any specs on voltage return values? SI I have shows only a resistance test.

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Adam Technician
Maplewood, Minnesota
Adam Default
   

Hahah! It all depends on what you are used to... :) i always went by approx 2.5v at 70ish deg. Down to under .5 fully warm. I would expect closer to 5v in your situation. whoops- I read 2-5 mpg down as you are getting 2-5 miles per gallon. Lol. I guess you probably don’t have a fully open circuit...but are on the right track.

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Bill Technical Support Specialist
Mesa, Arizona
Bill Default
 

70 degrees, man it will never be that cool (even at night) for a month or more. Last night's low was 87 IIRC Mileage on this thing runs a rock solid 20mpg, unless you hit the open highways. Never changes around town. Mileage has been all over the place for the last month or so. Hit as low as 15.3! Mostly in the high 17's and mid 18's I am liking the ECT being an issue but am having a hard time…

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David Instructor
Kokomo, Indiana
David Default
 

Hi Beevo, Definitely not "your father's Oldsmobile" kind of IAC! Great ideas from the other techs, especially those who have extensive experience with Volvo (which I do not). For some outside the box ideas (and at the risk of being booed by my DN peers) I'm linking a DIYer YouTube clip (guessing you may have watched already it but maybe not) from someone who appears to be an engineering student…

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Jeffrey Manager
Yuma, Arizona
Jeffrey Default
 

Interesting! If you read between the lines his poor TPS connection might have been the root of his particular problem. Thanks Dave,

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Bill Technical Support Specialist
Mesa, Arizona
Bill Default
 

I have yet to find the time to watch it. At this point I have come to a head slap realization, sitting in the driveway is my wife's '88 Volvo wagon. Not turbo, but I am sure any IAC signals will be the same. Sigh....

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