Air-fuel Adjustment '74 D-jet

[AlexO]

Since I have had it back on the road, my 914 has been running really rich from idle to 4500 or so. It is a '74 2.0 with a cam larger valves, and Bursch collector but stock EFI. My plugs are black, and i get a decent (not too big) puff of smoke if i snap the throttle off idle. I have set the fuel pressure a little low (26-27psi), and the CO screw seems to have no effect. Any ideas? --Alex

[bowlsby]

The ECU knob only functions to set idle mixture.

Have you checked your cylinder head temp sender resistance value?

Check here:

http://members.rennlist.com/pbanders/djetparts.htm

Or maybe your injector(s) or cold start valve are leaking?

Jeff Bowlsby
74 914 2.0L

[Jim Smolka]

Also, see how it runs with Temp sensor 1 disconnected.

[This message has been edited by Jim Smolka (edited 07-09-2001).]

[pbanders]

Originally posted by AlexO:
Since I have had it back on the road, my 914 has been running really rich from idle to 4500 or so. It is a '74 2.0 with a cam larger valves, and Bursch collector but stock EFI. My plugs are black, and i get a decent (not too big) puff of smoke if i snap the throttle off idle.

I have set the fuel pressure a little low (26-27psi), and the CO screw seems to have no effect. Any ideas?
--Alex

Pull a vacuum of 15 in. Hg or so on your MPS input port and make sure it isn't leaking (leaking = rich). What kind of cam are you using? Only the mildest improvements in lift and duration over stock will work properly with the D-Jet system. Bigger valves should increase volumetric efficiency somewhat - which the D-Jet ECU won't know about, so it would tend to make your mixture at a given engine speed (more air in @ same injection duration = leaner).

Low fuel pressure will cause a leaner mixture, since the injection quantity is directly related to the fuel pressure at a fixed injector flow rate.

I assume the "CO screw" you describe is the ECU idle mixture knob. Changes to this adjustment only affect the mixture at idle and during overrun conditions (where the engine speed is >2000 rpm). The knob has no effect at part-load or full-load.

Jim suggested checking how it runs with TS1 disconnected. If it's already running rich, disconnecting TS1 will make it somewhat richer. Shorting TS1 will make it run somewhat leaner. TS1 accounts for air density changes with temperature and produces a second-order effect on the ECU's calculation of the injection duration.

You should verify that you have all of the correct D-Jet parts for your engine as Jeff Bowlsby suggested. I'd also check on his suggestions about leaky injectors or the CSV. Note that if the control lead to the thermoswitch is touching any metal and is grounded, that the CSV will be activated and you'll get a rich mixture.

A final suggestion is that if you can't figure out what's going on, rent the VW 1218 tester from Pelican Parts and test the system out.

Brad Anders


[This message has been edited by pbanders (edited 07-10-2001).]

[pbanders]

whoops, accidentally posted this twice.. edited it out....

Brad Anders

[This message has been edited by pbanders (edited 07-10-2001).]

[Dave at Pelican Parts]

Actually, Brad, the CSV gets its power from the yellow starter wire. That means that, unless it is leaking, the only time it can open up and dump fuel into your plenum is if the starter motor is currently cranking. So, even if the wire is shorted to ground, the car should run OK. It just won't want to start when warm.

Ask me how I know this...

--DD

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Pelican Parts 914 Tech Support

A few pics of my car: http://www.pelicanparts.com/gallery/Dave_Darling

[pbanders]

Originally posted by Dave_Darling:
Actually, Brad, the CSV gets its power from the yellow starter wire. That means that, unless it is leaking, the only time it can open up and dump fuel into your plenum is if the starter motor is currently cranking. So, even if the wire is shorted to ground, the car should run OK. It just won't want to start when warm.

Ask me how I know this...

--DD

Duh. I'm really losing it, you're absolutely right. I need to start reading my own web references before responding !

Brad Anders

[pbanders]

Follow-up question - DD, do the CSV's ever really leak through the spray port? Or is this one of those 914 things that never really happens (e.g. like a failed fuel pressure regulator)? I think you or someone else here reported on a leaking CSV valve at the fitting, but not the sprayer.

Brad Anders

[Dave at Pelican Parts]

I don't really know for sure. I have heard of it happening, but it's never happened to me, and I can't think of anyone who has specifically mentioned it to me.

My old CSV had a wobbly plastic part (the fuel fitting and electrical connector), so I pulled it and replaced it. I don't think it was leaking around the pintle, but I don't really know.

--DD

------------------
Pelican Parts 914 Tech Support

A few pics of my car: http://www.pelicanparts.com/gallery/Dave_Darling