Very Rough, Low Idle and Missing - Help!, 73 D-Jet - SOLVED! Options

[ThinAir]

RRC is only a couple of weeks away and I'm beginning to get desperate that I'm not going to be able to drive my 914.

When I posted this, my problem was just that the idle would hunt, but otherwise it drove well. After reading the responses and reviewing Brad Anders' stuff again it made sense that my hunting problem was probably that I was just running lean. I turned the ECU 3 clicks clockwise to see if that would improve things, but it didn't seem to.

Well, things have gone from poor to bad. Right now I've got a car that barely idles at all, and when I'm accelerating or driving on the highway it often bucks/misfires. I haven't run it long enough to get it warmed up. I've tested the AAR by pulling the supply hose and it seems to be working. If I remove the AAR output hose the RPMs go up to 1000 so perhaps the AAR isn't opening fully.

I've measured the ohms on the CHT at 124 so that's within range, and I've got a brand new 270 ohm resistor inline. I have 3 MPS units that all pass on resistance and vacuum and changing them makes no difference in the behavior. I've also recently replaced the spark plug wires (not to solve this problem, but just because) and I've made sure that they are as far away as possible from the MPS unit. I've also checked every electrical connection that I can find.

Sorry that all that is so long - any clue what I should be looking at? I'm beginning to panic (IMG:style_emoticons/default/blink.gif)

[ThinAir]

Update: Tonight I pulled all the spark plugs, checked the gap and cleaned them. They weren't fouled, but they were sort of a darkish brown and certainly would benefit from a little treatment with a wire brush.

Once I fired it up, it ran pretty much just like before. I took it out for a drive so that I could get it warmed up. It sputtered and bucked, but after about 10 miles on the freeway it suddenly smoothed out. I drove it another 30 miles at 75 mph, even tested acceleration from 75 to 95 and everything was smooth as can be. Once I got home it was idling smoothly at 900-950 just like I want it.

So I'm pretty sure that this is a cold-engine issue. The proof will come tomorrow morning when I drive it to work. Work is 8 miles on city streets and takes 20 minutes. I've long suspected that this does not work the engine hard enough to truly warm up.

So... this is gonna be a bit long, but let me give you a rundown on this car and the history of the problem I'm trying to solve. There's a question at the end, really!

This car is a 1973 2.0L with stock D-Jet fuel injection. All parts match Brad Anders list of 1973 parts, including the CHT and the home-built 270 ohm inline resistor. I've got Bowlsby ignition and FI wiring harnesses that are 3-4 years old. Battery is in good condition and alternator light does not come on at idle (after recent adjustment of the alternator belt). McMark rebuilt the distributor last summer, including the injector points.

At WCR 2011 I asked McMark to drive my car because I was having a problem with bucking when I went through 15mph school zones in 2nd gear. Otherwise the engine was running wonderfully. He diagnosed a MPS issue, which was proved by swapping the MPS with a known good unit. After I got home I traded for a known good MPS, but once installed I had the same issue. With help from the World community I was able to determine that this was due to the routing of my spark plug wires. I recently changed to 8mm Clewitt plug wires in hopes that it would reduce the chance for interference with the MPS.

I wanted to put my original MPS back in because I just trusted it more, but when I swapped MPSs the engine bucked and would hardly run. Of course this was strange since I'd run that MPS for a long time, but I'm not one to argue with reality so I put the other one back in and the engine went back to running pretty much as I'd like.

Somewhere along the way I noticed that the engine wasn't idling well, and I opened up the throttle bipass to get it to be less annoying, but clearly there was a problem. I started reading through Brad Anders stuff again, particularly his article on stabilizing the idle, and decided to do exactly what he said - check all the basics first, then run the car for 30 minutes on the highway to make sure it was well warmed up before setting the idle. I also turned the ECU knob clockwise by 3 clicks to slighly enrich the idle mixture.

That all went well enough, and I had a warm engine that was idling at 900-950, until I went to back it up to pull into my driveway. For some reason it started sputtering and eventually died, refusing to restart until it had set for several hours. It made me think it had flooded.

Later I had run it again until it was warm in hopes of adjusting the ECU knob to get the idle as smooth as possible. I had moved the battery to get to the ECU knob, and when I moved it back the engine died and would not restart until several hours later. Again, it figured it was flooded.

I noticed that the CHT lead with my home-built resistor was laying across the positive battery cable and had moved when I moved the battery, so this made me suspicious of the CHT resistor and I built a new one. Pelican's site says that a bad CHT sender circuit can result in just the kind of behavior I've been seeing. I try to be careful when handling hoses & wiring, but I'm wondering of something got damaged when I was swapping around MPS unit. I can't see anything and the CHT measures the right resistance, but they can just wear out also. Running this new resistor has made no difference in the rough running.

So I decided that I should have a spare CHT, even if the existing one wasn't bad. Pelican lists one for all years, except 1973, and says the one for 73 is NLA. (I haven't been able to find a 0 280 130 003 anywhere.) GPR lists a 0 280 130 012 unit as being for all 914s so I ordered one of these. Brad Anders makes statements about CHTs in several places that mention conflicting part compatibility and in one place makes it sound like the 0 280 130 012 unit will work with a 73 & 74 2.0L, but with no mention of whether you run it with the 270 ohm resistor or need some other value.

So here's the question (besides, do you have any ideas about what's going on): When the 0 280 130 012 arrives, can I run it in my setup with the 270 ohm resistor, or do I need another value? I don't want to do any harm or have a different issue to chase.

Thanks for your patience if you read this far. At least you were warned that it was long ;-)