740 Volvo IAC issue
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
What year is it? Engine size?
Im assuming there is no vacuum leaks?
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.
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
I see there are 2 control circuits to the ecm, is this valve a stepper motor?
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…
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.
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.
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…
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.
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.
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,
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…
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…
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.
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.
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.
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…
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…
Interesting! If you read between the lines his poor TPS connection might have been the root of his particular problem. Thanks Dave,
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....