D-Jet diagram? Help...

[51coupe]

I just purchased a 914 from a reputable 914 source (across the country). When it arrived it ran somewhat crappy. I figured it was something simple as the shop I bought it from knows what they are doing and put a bunch of time and new FI parts into the motor... and the owner took it on a trip.. so I have been looking for a red flag.. well, I found a plug in the FI wiring harness that was unplugged (SEE PICTURE its on there passenger side, two prong plug, now plugged in to the plenum?). It looks like it fell off in transit? Now with it plugged in the car barely runs and I do not even know what this plug is.. I have searched and all I can find is either schematics or vacuum hose diagrams. Is there any decent D-Jet diagrams that show the FI components and corresponding wiring harness attachments? I need a starting point to know what the hell I am even talking about..

How could it run better with a necessary connection unplugged?

[51coupe]

Here's a better picture of the sensor that was un-plugged..

[beatnavy]

Having a hard time seeing what you're pointing at. Tell us more about the car, if you can (e.g., what year). I assume it's a 1.7L based on the shape of the air cleaner.

So I don't know a single FI diagram off the top of my head, but there a couple of great resources for learning about how D-Jet functions - both on 914's as well as other cars of the era that used it:

1. Anders D-Jet Parts and Troubleshooting (I have been on this site probably 1000 times). If you're going to own and maintain a D-Jet 914, you need to bookmark this site.

2. D-Jet Articles (run by a guy in Germany). Good info, more theoretical and not specific to teeners.

3. Jeff Bowlsby's Tech Notebook. Has some really good guides on there about how D-Jet works and how to troubleshoot each component.

To your specific question, you are PROBABLY pointing at the T1 sensor (I'm guessing based on location and two prongs). That's the ambient air temp sensor. Some mechanics disconnect that to make the car run slightly richer. An open circuit there signals "cold air" and the ECU somewhat richens the mixture. If your car barely runs with it plugged in then you are probably way too lean. When you say it "barely runs" what is the actual symptom?

[beatnavy]

Based on your second picture, yes, that's the T1 sensor. Tell us more about it barely running. And yes, you've got a 1.7L.

[51coupe]

Sorry, I should have been more clear.. The car is a 1973, 1.7L. It has 82K miles.

The behavior when the sensor is plugged in:

Car starts right up but then the idle "hunts" up and down, not reaching a steady RPM. When I attempt to drive the car, it is rough and ragged, feels like it is missing a bunch. If I open the throttle it gets worse than if I feather the throttle at super low throttle openings. This is similar to when the sensor is plugged in only 1000-times worse. Think "bucking and kicking"..

Thanks for any input. It is greatly appreciated. I just need a starting point. D-Jet is all new to me but I have been reading up on it, I just frankly know what I am looking at half the time. The resources tell you what this or that component do and their failure modes but I am having trouble understanding what the components are in my engine bay that I am looking at...

Regards, Peter

[51coupe]

I just measured the resistance and the T1 (if thats what it is, seems to be in spec - 315 ohms at about 65 degrees)

[beatnavy]

No worries, Peter, I know what you mean. I've been futzing with my D-Jet for the better part of 3 years now (which I enjoy, in a puzzle-solving kind of way - except when I'm stumped for a while ). Pretty soon you'll know all the pieces by heart and what causes what. The challenge with sorting out a D-Jet system that is old / poorly maintained is there are typically multiple variables in motion that make troubleshooting difficult. After a while, though, you solve them one by one and it starts running like a champ.

Anyway...yes, you've got significant lean condition (as one issue). That idle hunt is evidence. Someone did the "mechanic's trick" of unplugging the T1 to make it richer (and mask the root cause). I would recommend reading Anders site there based on your lean condition.

Also, running "rough and ragged" and "bucking/kicking": could be the lean condition, or it could be cylinder or injector not firing reliably. Outside the ignition system, the D-Jet components here to look at are the Throttle Position Switch (TPS), the trigger points, and the injectors/injector plugs.

Again, a LOT of variables that you need to eliminate so you isolate your problem. Start with basics like:

1. Setting dwell properly (45 to 50 degrees)

2. Verifying timing (27 degrees btdc at 3500 rpm)

3. Making sure fuel filter / lines are not clogged

4. Verifying fuel pressure is ~29 psi

5. Adjusting valves when you're up for it

Lots to do. Just approach it methodically. All 5 of those things are good to do no matter what the actual problem(s) are. We can point you at resources for how to do all this stuff.

[51coupe]

Thanks so much for taking the time...

When I got the car it had a freshly rebuilt fuel pump four new Bosch injectors, a new air temperature sensor, and a rebuilt pressure sensor. Valves adjusted, timing set, fuel pressure set to 28 pounds. Usually I don't believe people when they say they have done all the things they say but in this case I do..

Thats the good part. I did measure the cylinder head temperature sender when I first got the car and tried to come up with a idea of why it was running rough especially when cold (given the little knowledge that I have).. and the CHT sensor is out of spec... something like 3400 ohms @55 degrees and 130ohms hot. I have a new one on the way.. won't a good one make it run even leaner though?

The car was also on a transporter for a week and they could have left the keys on for a while, Could that have done something to the points enough to exacerbate the problems?

I should emphasize that before I plugged in the T1 sensor, the car ran sort of crappy when cold but much better when hot. Full throttle hot was almost in the realm of good performance... It was a drivable car when cold and a decently drivable car when hot...

I'll check the fuel supply path and see what I come up with but it seems odd that the car would run better on full throttle (when hot) than at part load if it had a fuel starvation problem..

Thanks for the help.

Peter

[beatnavy]

Ok, that's good additional info.

Assuming your ignition system is healthy (timing, spark wires, plugs, dwell, etc.) and your fuel system is as well (fuel pressure, filter/lines clean, injectors not clogged), then "bucking and kicking" could be indicative of:

1. A worn out TPS board. TPS have little electronic traces on a primitive circuit board. If the computer can't detect s smooth "on/off" pattern as the throttle moves across them you'll get bucking under acceleration. You can remove the TPS and inspect the traces. Some people get at least temporary relief by gently rubbing them with a pencil eraser, or carefully bending the contacts to create a new path across the traces. You can also do a test as follows: with the ignition on but not running, manually (with your hand at the throttle) and slowly open the throttle. If it is quiet enough in your garage you should hear a little "click" as you move it. There should be something like 20 total clicks, IIRC, that click at regular interval as you move throttle position from idle to WOT.

2. Trigger points worn or dirty. This requires pulling the dizzy, so you may want to do the "easy" things first. They are sort of "inside" the distributor and they are key to firing the injectors at the right time.

3. Injector connectors not snugly seated.

4. Worn out wiring, connections, dirty grounds, etc...

5. Finally, verify vacuum hose connections and seals (e.g., injector seals). Always a good idea to eliminate vacuum leaks as a variable.

If it runs better when hot I'd potentially throw the AAR in there as well. It may be frozen shut. That would be more like a "low idle" rather than an idle hunt, but these do fail frequently as they age and get dirty.

The CHT is an interesting beast. I don't think yours "out of spec" is the root cause. Make sure you're not shorted there as that may cause things to be lean, particularly when the engine is cold. Ensure the wire lead is not hitting the block. Removing and installing the CHT can be a little tricky due to its location, so be careful. More than one case has had its CHT hole threads stripped requiring a creative solution or repair. My experience with CHT's is usually an open circuit, which causes a much richer mixture to the point the car won't start unless stone cold and then only once for 30 seconds or less.

Leaving the key on while transported? That will easily kill something like a Pertronix ignition. Not sure about stock. Maybe it could it fry the coil or points. Go ahead and inspect the points.

[911buff]

Definitely check the points. We struggled to eliminate the bucking; rebuilt harness, new CHT, new TPS board, rebuilt injectors, on and on. No matter what we did, it still ran rather rough and stumbled and bucked. On a whim, I decided to swap out the electronic points for an original set. Now it runs like a dream, smooth as silk with no idle issues. I want to purchase a new electronic ignition pack but in the mean time the original points are the ticket.

So, it might be worthwhile to purchase a new set of points and see what happens...cheap trouble shooting (it also eliminated the tach bouncing). Just a thought.

[51coupe]

Ok,

So I did what my ability and time would allow then I handed it off to a shop. The guy diagnosed the Auxiliary Air Regulator as bad, adjusted the valves, checked the timing, checked over the vacuum lines.. he replaced the Auxiliary Air Regulator with a decent used unit .

Unknown to me, the Air temp sensor was unplugged when I bought the car and when plugged in the car would barely run. Now with the new (used AAR) the car runs ok with everything basically as it should be.

While the car idles fine when cold or hot I am still having some bucking when cold. If I feather the throttle is smooths out a little and if I open the throttle wide it seems to help. When hot, the car runs pretty well. Also, when I say cold, I mean California cold... like 55 degrees.

One problem is that the head temp sensor on the engine and is out of spec (higher reading than normal - both hot and cold). I am afraid that if I change it out the car will run even leaner (I am assuming a lean condition is my problem) so I still have some issues to deal with... I checked and it does not seem like the lead to the sensor could be shorting against anything as far as I can tell..

Any ideas to get the car that last little bit closer to where it needs to be?

[beatnavy]

Have you verified fuel pressure at or around 29 psi? You may increase, very slightly, to like 30 or 31 (don't go much higher) to enrichen things. You can also disconnect the T1 sensor which will make the mixture richer. These are, at best, band-aids, not fixes for the root cause.

It is possible your T1 sensor, or the leads to it, are shorted.4

Bucking can be caused by lean condition or bad/misadjusted TPS. Because it happens when cold, I'd focus on that T1 sensor though.