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#013 AMM

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Discussion Starter #22
I think i might have it fixed! Even though the NTC sensor looked good on resistance checks at different temps i decided to swap it out with a spare i had. Went through 2 iterations of cool down and back up to op temps where fans kick on. Idle stayed normal and engine ran like normal. Goosing the throttle seemed good as well. Also, the CEL went out within few minutes of the first start up and has stayed off.
Might be a little premature but looks like it was the NTC.
I'll post back when i know for sure. thanks!
 

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Discussion Starter #23
Update: Belay my last. Issue not fixed. Did another run up after cold. started horrible. Had to keep feathering throttle to keep it going after few minutes it calmed down and started to idle somewhat normally. Before the fans could come on, around 160F, Idle increased to 13 or 1400 and then started hunting.
Also, I used my vacuum pump to connect to each vac line individually (motor off) to see if each branch would hold a vacuum. They all did accept for the Charcoal cannister line (ELCD). I'm assuming this is regulated air and should be that way with engine off. Doesn't seem to affect idle with it plugged.
I guess i'm going to wait for the used AMM from ebay, see what that does.
I've also got my feelers out for another ecu. This time (yes, this will be my 2nd ecu purchase) instead of the 0280000585 which is for 1990, gonna look for a 0280000948. From what i a glean during searches these were preferred over the earlier 585.
 

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I am pretty sure the charcoal canister should hold vacuum. The purge valve should be shut tight when power is taken away. It could be the source of your problem, since the purge valve is run only under certain circumstances. Plug that line, see if the problem goes away.

Edit: To clarify, the LINE between the manifold and purge valve should hold vacuum. The canister itself is obviously connected to the tank.
 

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Discussion Starter #26
I plugged the vacuum port for the ELCD line at the maninfold - no change. She warms up to about 160f or so and starts idle hunting.
 

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Lacking any other obvious avenues a bad MAF is on the table... they are a mystery box we don't get visibility into. IME they are ALWAYS wrecked on c900s but rarely does anybody notice until the weird problems show up, so you could be there.
 

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Discussion Starter #28
Ya....i did do the Bently test where you run the car up to op temp then shut it off, look into the AMM barrel to see if the wire glows red for 1 second; it does. At that point Bently says it's prlly ok then so i stopped there since i have another coming this week.
 

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IME the burnoff test is about useless. It just shows one function of the MAF works, but says nothing about its primary function of actually metering air. I think Bentley included it just so they could have a test.
 

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Discussion Starter #32
Ok. I'll keep my eye on it. I"m in the market for one. Mine is leaking oil internally pretty bad. It's one of last two oil leaks i have. The other is the shifter rod seal in the transmission which is next on my list.
Concerning the this thread, i'll post back results once I get the used AMM. I also bought another ECU for just in case.
THanks, jv.
 

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Discussion Starter #33
Lacking any other obvious avenues a bad MAF is on the table... they are a mystery box we don't get visibility into. IME they are ALWAYS wrecked on c900s but rarely does anybody notice until the weird problems show up, so you could be there.
JV, not sure i know what you mean by "wrecked". Are you saying because of the age of our c900's the AMM's on our cars are always bad or not operating properly?
 

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Discussion Starter #35
I got the used AMM from ebay last night. Installed and seems to be the fix. Car started and sounded great. Idled normally all the way to when the fans turned on and afterwards. Took car for 20 min drive and all was normal.
Looks like was was the AMM (MAF). I'll update if the problem surfaces again.
Thanks everyone!
 

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It's really amazing how well a c900 will run even with a MAF that barely functions. I used to keep a special MAF in a box and use it only during smogs. With it, the car couldn't make any useful boost, but otherwise ran well enough and it would sail through smog without an issue.
 

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Discussion Starter #37
It never left me dead on the road and it really ran pretty good at highway speeds.
I will say that the symptoms did not remain constant. Sometimes it would run horrible after cold startup. Other times it would start up cold fine but run bad after warmup.
On a side note, JV - ship it:)
 
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