Adjusting Fuel Pressure for Better Running Engine, 1.7 Engine Performance Options

[R Shaff]

I've read some helpful things here while helping with my brother's '72 914. I think I've learned something new and thought I'd share it with you guys.

This car has the original engine and FI system, though the ignition system has been updated and is electronic. The car wasn't running well, and the local shop my brother uses said he should do a carb conversion, costing over $4,000. We looked online and found some good write-ups that recommended keeping the FI system. I've worked on many cars and restored one, so I thought I'd try to get the car running well enough with the original system.

The compression and ignition checked out fine. We adjusted the valves (of course). We tested and then replaced 3 injectors, and the CHT sensor. I put the fuel pressure at 28 psi, per spec. Static tests of the manifold pressure sensor came out fine. A cigar smoke test showed no intake leaks. We cleaned and adjusted the TPS, and then finally changed the TPS contact plate and adjusted the arm (when the car just wouldn't run right). All the other components checked out fine, using tests I found online.

The result? The engine fired right up, with a good idle. But when driving it lacked power, and the higher the rpm the less it wanted to pull. (Not right for a Porsche.) It felt like the timing wasn't advancing, or that the engine just wasn't getting enough air or gas in the right balance.

Other than buying a replacement MPS or computer, I was running out of ideas to try. One of good things about carbs is that they are adjustable. One of the challenging things about these old open-loop FI systems is that they are not really designed to be adjusted much, or at all. Repair is usually a process of testing and replacing components. But this car sure felt like it needed some kind of adjustment.

One of the odd things about this car, in my experience, is that the fuel pressure regulator IS adjustable. So that's what we tried, just using seat-of-the-pants performance as the indicator. We dialed the pressure down a turn and things got worse. Then we turned the pressure up, one turn at a time, and voila! As we did the car ran better and better. Now it pulls in the upper ranges like it should, much like it used to.

Now we're going to buy a 4th injector of the same type and adjust the pressure again as needed. (Now that we know we can adjust the whole system.) Perhaps that will make it run better still.

I haven't read that anywhere(about adjusting fuel pressure to get the engine to run better) so I thought it might help someone else who is struggling with these old Bosch systems.

Happy motoring!

Richard

[R Shaff]

OK. I'll check and prob clean the advance mechanism. Thanks.

Moving now from facts to personal opinions... I have a pressure gauge but I probably won't be using it for this anymore. For starters, it's a pain to do and usually gets some gas in the engine bay. Second, it's real easy to take a couple of wrenches with me, go for a test ride, and tweak the fuel pressure at stops along the way.

When making the mixture richer, with higher pressure, my plan is to take it only as far as it makes a noticeable difference. No further. And I plan to check the plugs relatively soon to make sure that setting isn't too rich overall.

Basically, I think every mechanism works best with some form of adjustment. The more complex the mechanism is, the more it needs it. None of these engines are new, and the components have unknown degrees function, misfunction, and variation. Modern closed-loop systems adjust for those variables constantly. These old cars are the opposite of that.

I don't know whether the engineers at VW/Porsche planned for the adjustable fuel pressure regulator to be used this way or not. (I suspect not). But for a car this old with this many variables, having an effective adjustment capability sure helps!