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1985 Saab 900 Turbo
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'85 Turbo sedan (2.0L/auto)

When starting the engine cold, it stumbles and will not idle without working the throttle to keep it from stalling. If I start it cold and hold the engine ~2000rpms it will stay running without stalling, but if I let off the throttle it will stall. It will do this for maybe 30-45 seconds then starts/runs fine until the engine is cold again.

If starting when the engine is warm it starts/idles without issue.

Any thoughts where I should start troubleshooting this issue?

Thanks,
Dan
 

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I suspect my brake booster is bad and leaking vacuum, but I also checked my MAF and attempted to adjust the resistance to the 380 ohms starting point (is currently at 1050 ohms), but turning the adjustment screw didn’t change the resistance at all. So while I think I have a vac leak, I also think I have a bad MAF
 

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I know that 380 ohms has been floating around for a long time, but I'm not sure what it's based on. The actual procedure for setting the idle mixture is feedback based - using the O2 sensor or a CO meter etc. - not configuring for a specific value. The general idea is to get the engine warm, then dial in idle mixture by observing a sensor and adjusting the MAF as necessary to get mixture where it should be. I imagine in 2021 we could use an AFR sensor, too. Values will no doubt vary from car to car - wear & tear and environment etc. will definitely have some sort of impact on idle mixture. LH 2.4 eliminated the need for this adjustment as it learns (infers) idle mixture from operating mixture.

That said, if turning the screw doesn't move you from 0-1000ohms, it definitely seems like the MAF is bad. :)
 

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1985? All other common issues have been addressed already? Affects only cold start?
Add a 15 ohm resistor, in-line, to your NTC sensor circuit.
 

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@jvanabra, yeah I know that 380 ohms is often misidentified as the end all, be all target resistance but it’s really a starting point. My O2 sensor appears to be working fine, fluctuating between 0.15 and 0.8 v at idle when engine is warm. Per Townsend, voltage drops closer to 0.1 when you pull a vac line and up to 0.8 or just above when I rev throttle. So perhaps my MAF at 1050 ohms is ok for the car, but it still doesn’t adjust with the screw and the car stalls immediately at cold startup.
@Jim Mesthene, all that I’m aware of, per Townsend. I adjusted timing to 16 BTDC with distributor vac line plugged, new spark plugs, cap, rotor and ignition wires, tested o2 sensor, cleaned throttle body and plate. TPS didn’t appear to need to be adjusted since the continuity test passed and it clicks as soon as I open the throttle. The only other issue is my bad brake booster, which I just confirmed doesn’t hold vacuum. Should that be addressed before replacing MAF? Also, what is the NTC circuit and where would you recommend adding the resistor?
 

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Definitely fix the booster first. A big vacuum leak like that will lean things out and make cold start even more difficult.

Doesn't matter which "side" you add the resistor to.... all you're doing is tricking the ECM into thinking the engine is a slightly colder, so it will add slightly more fuel.
 

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i’ve ordered a couple used MAFs, so we’ll see if those help first. If not I might try the resistor work around. If so, I’ll be back here asking for more guidance from you @jvanabra and @Jim Mesthene . My knowledge of electronics is very slim currently. I wouldn’t know where to add the resistor in the car (or how truthfully)
 

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Update here, with some improvements and also new issues:
  • Good news: replaced AMM with another 005 unit, and car started right up cold and didn't stall immediately. Some bouncy idle at first, then settled down to 1000 RPMs (I know there's a vac leak in the brake booster; ordering a reman'ed one). Idled smoother than ever (engine was hiccupping/misfiring with previous AMM) while engine was cold/warming up and also when engine got warm (fans cycled 2 times).
  • Bad news/new issues
    • Hesitation/no acceleration: opened throttle and heard/felt significant hesitation right away. Took car out for test drive, and it was even more noticeable. Car would almost stall if i accelerated too quickly from a stop; engine surged and jolted as I accelerated, eventually topping out at 2500 to 3000 RPMS. Virtually no turbo boost. I tried merging on the highway to see if symptoms different at higher speeds/higher gear, but acceleration was so weak that I couldn't even merge onto the highway safely because car was going so slow.
    • O2 sensor - checked this while engine idling warm, and got no fluctuating voltage, just a steady 0.7 volts. Voltage did drop slightly when i opened throttle (0.55 or 0.6v). Adjusted the AMM adjustment screw to 380 ohms as a baseline to see if that'd help, but o2 sensor gave the same non-fluctuating voltage. Checked preheater circuit and getting 14v there. Bad o2 sensor sounds like, right?
    • Bouncing RPMs as engine settles into idle: less of a concern at the moment, but could this be a bad/sticky AIC? I've checked resistance at the 3 pins and they're all spec, and it's getting battery voltage when car is idling.
Any ideas about the hesitation? Would a bad o2 sensor cause this?
 

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If the car is running lean (due to a vacuum leak or any reason) there will be hesitation at tip in. Older versions of Jetronic do not have a load sensor (via TPS) so there is no mechanism for throttle enrichment... If it's already running lean it will be worse when you open the throttle.

Steady voltage from the O2 sensor could be part of the problem. Assuming there isn't a wiring problem, it's probably a defective sensor which should be replaced.

I would fix the things you know are broken before worrying about other things that you don't know. ;)
 

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If the car is running lean (due to a vacuum leak or any reason) there will be hesitation at tip in. Older versions of Jetronic do not have a load sensor (via TPS) so there is no mechanism for throttle enrichment... If it's already running lean it will be worse when you open the throttle.
My car is 1985 900 turbo running LH 2.2, so not jetronic

And my thoughts too about the sensor. I have ordered a replacement O2 sensor.

A couple follow up questions
  • O2 sensor fluctuating voltage: higher voltage means running richer and lower voltage means running leaner, right?
  • Relationship between AMM adjustment screw and O2 sensor voltage: is the idea to adjust the AMM screw until you start to get a fluctuating voltage? Or, said another way, can the AMM resistance be adjusted to a point where the O2 will start fluctuating voltage? Or should the O2 sensor produce a fluctuating voltage no matter the AMM resistance?
 

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"LH 2.2" is LH-Jetronic 2.2. :)

Jetronic was Bosch's trade name for electronic fuel management. Eventually it disappeared and Motronic took over, their complete engine management system.

I don't know whether it's possible to mess with the AMM to the point the O2 sensor isn't working properly, but I kind of doubt it. If it's stuck at one voltage, it means the ECM is basically not managing fuel at all OR the sensor is broken.
 
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