Hunting down the M50 idle issue.

[SnakeyesTx]

Since I've gotten this new motor, I've put about 200 miles on it. It was recommended that I run it for a little while on the highway to clear out the fuel system and carbon since this was a salvaged motor that may have sat in storage for a while. It runs "okay" and its nice to feel the Vanos working, but my takeoff is a little slow till 3500 rpm.

My biggest issue is I have a lumpy idle. Sometimes when I sit at a light, it idles poor enough to make the car buck ever so slightly. Initially I traced some of the idle issue to a bad coil. The sides of the pack had high temperature burn marks and a neighbor had heard that coil arcing to ground. We tested this by switching coils and the arcing noise followed. After replacing the coil the idle was fairly smooth for about 10 miles then came back to its slight bucking thing at lights and poor idle quality. It idles at about 750 rpm.

I took the new plugs out (NGK #2288) after about 4 days of driving 28 mile round trips to school and back. The plugs had no black residue from a misfiring or over-rich condition. Instead all 6 had a little pale yellow scale on the grounding electrodes which easily wiped off. I noticed the ceramic insulation around the electrode was also ever so slightly yellow. So far these things don't really shock me as odd, but I also noticed the ceramic of the exterior of the plug around the hex-headed base also looked a little yellowed too. I began to wonder if perhaps the plugs I purchased were the wrong heat range so I consulted NGK and they verified that these were the proper plug and heat range for a stock M50. I also checked the big 3 local parts houses in town to make sure, and all their numbers matched as well.

So I don't have a bad coil anymore, but I still have the idle issue and I moved on to the ICV possibly in need of cleaning. Looking around realoem by my vin number, I'd have to say that the diagram they have listed looks similar to mine, but I can't find this T-shaped Idle regulator valve they have shown for my car. There was a little soot around the throttle body venturi area so I shot some 3M Throttle plate and choke cleaner in there and it cleaned right up. I took the 90 degree elbow off from the throttle body to the Mass Air Flow sensor, and noticed that there were two huge lines attached to it. One was big enough for me to fit my thumb into it and it attached to a green plastic fitting into the elbow. The other one was slightly smaller than my pinky finger (I'm referring to the actual inner diameter of these hoses). The smaller line goes to the front of the head (assuming crankcase ventilation tube), and the big one disappears into the bowels of the engine (about center of the motor well under the intake).

With the elbow off I started the engine and it ran VERY smooth and nice. I stick my thumb into the larger hose and it has MASSIVE vacuum! It sucked my thumb in tight and the motor stalled. Okay, so this is obviously a vacuum line that feeds into the throttle body and also aids the crankcase ventilation hose. I noticed a little tiny oil residue between the two elbow fittings.

Anyway, without the elbow with its two hoses it idles pretty nice and smooth but after about 10-15 seconds the motor stalls out. I'm guessing that its because the MAF isn't getting a reading of air going into the motor. I put the elbow back on, start it again, and the idle returns to poor. In a last ditch check, I unplug the MAF with the elbow already back on. Idles smooth and nice again, albeit about 1500rpm. I know you can test MAF's at idle by unplugging them and either the idle smooths out or the engine stalls, but if it stays running, you can't really drive without it since it won't run very well. As soon as I plug the MAF back in, idle goes back to poor.

Now the car does drive as is like this, but usually when a MAF goes bad, in my experience you can't really drive it at all. Does this actually sound like a case of a failing MAF? I'm kinda running out of ideas on what to check next.

[ryan roopnarine]

the large one that goes from the elbow to under the intake manifold is the one that connects to the idle control valve. if that hose is obstructed (by your thumb) or disconnected, the engine will only run for a few seconds before dying. the idle control valve sits in a rubber hanger under your intake manifold.

[SnakeyesTx]

Awe man.. that's gonna be really hard to get to I wonder if I could just spray a little cleaner down the hose to clean the ICV without having to muscle my way down there to take it out.

[ryan roopnarine]

Awe man.. that's gonna be really hard to get to I wonder if I could just spray a little cleaner down the hose to clean the ICV without having to muscle my way down there to take it out.

it won't help you if it is stuck, as sounds like is the case with many on the board (i've never seen that phenomenon). it is a PITA the first time, but if you do it enough it isn't that rough to get it out or put it back in.

[Ross]

Ha,Ha the ICV on that motor is a real bitch to get at. Your plugs sound fine.

[SnakeyesTx]

Yeah, I'm thinking ICV now. I swapped MAF's with milkboot's and the car ran the same with the lumpy idle. Is it possible to get at the ICV from underneath the car on ramps?