FI connections

[Dave Thompson]

I recently purchased a '74 2.0l and drove it from So Cal to Twin Cities the first week of January! Without the heater connected. That's a tale in itself, but the main cause of my chagrin along the way was trying to figure out which of the FI connections was most responsible for choppy running and how to know what to connect or not. I am curently running without the connector attached to my throttle switch. i have read the tech forum article on adjustment, but haven't gotten to it yet. I have also disconnected the twin wire from the small vertical socket just to the left of the throttle intake, beneath the air filter. I don't know what this one is, but with it and the throttle switch attached, my car bucks and chops so bad whenever I release the throttle at or near 3000 RPM that it nearly drove me nuts all the way across Nebraska! I know everyone seems to feel the Haynes manual is great, but quite frankly, I think it is very inadequate at pointing out which connection is which, and worse at explanations, as it virtually skips theory of operation (one of the most important aspects of a tech manual) and only deals with most rudimentary adjustments. Can someone shed some light on these two connectors and help me figure out whether or not I am going to ruin my engine if I don't use them. Are they just part of a crude emissions/economy effort? Help is needed here. Thanks

[John Rogers]

I might as well go first....On this site and others is the article on the FI trouble shooting and is invaluable, find it. Also look at the post a while ago for "high idle" for the address of a scanned copy of the 914 FI systems. Both connections can be disconnected, the throttle switch might be out of adjustment or dirty, etc since disconnecting it makes it run better. The other is a temp sensor and ours is disconnected and causes no troubles.

[Dave at Pelican Parts]

First, on the Haynes manual. Yes, it is woefully inadequate for diagnosing FI problems. Yes, it is just as bad for many other things. But--it is the best single manual out there. It covers more than the other aftermarket ones (as far as 4-cylinder cars go), and is a heck of a lot less than the factory manual, which isn't that good either.

Next, recommendations for further reading. I think Kjell Nelin's D-jet FI article on the 914 Fan Page www.914fan.net in the Tech Refs section identifies most of the components and some of the theory. Even better is Robert Probst's book "Bosch Fuel Injection and Engine Management". LOTS of theory and descriptions of how particular components work. Not so good on identifying exact placement of components or testing, but very solid on general theory. (Too bad the D-jet stuff is shoved to the side in an appendix.)

Next, your connectors. The one on the manifold, near the throttle body, is the intake air temp sensor. Disconnecting it tells the ECU that the air coming into the engine is "very" cold, so the ECU commands more fuel to be squirted by the injectors. The effect is actually fairly small in terms of the actual fuel mixture. This is an old VW mechanic's trick to get the mixture a little bit richer.

The car will run just fine without the throttle position switch. Acceleration will suffer, though, as the TPS has traces inside that tell the ECU to squirt extra fuel when the throttle opens, much like an accelerator pump in a carb. Also, the ECU never gets into "idle" mode, which means the knob on the ECU should never affect the fuel mixture.

Finally, a general point about FI system troubleshooting. If you suspect an FI problem, ALWAYS ALWAYS ALWAYS check your ignition system. Examine the points (if you still have them), check the dwell and timing, make sure that you have a good fat healthy spark. THEN go on to troubleshoot the FI.

Get the Probst book. I'm pretty sure it's available through Pelican. (BTW, I *hate* the book "How to Tune and Modify Bosch FI". The sole premise appears to be that changing the air flow meter is the only way to make more power on most cars. Stupid.) Download and read Kjell's article. That'll give you another take on theory (I'd believe the Probst book over him on theory, though!) and reasonable specifics on a 914 engine.

Hopefully that'll be enough to get you going.

--DD