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]

Thank you, all. I appreciate the input.

My reactions, for what they're worth:

First, we don't know that the MPS needs adjusting. It might. Probably does. It's only 45 years old. I wish I could do it easily. From what I've read, I doubt I could do it thoroughly or well, even with a lot of effort. (Taking it apart sounds like a daunting task.) If someone has a good way to adjust it and test it properly, without running the risk of screwing it up, I'd love to hear it. In short, if I can get the car running well enough without risking the MPS, I'll probably do that.

Second, even if the MPS could use some adjustment, I don't agree that I shouldn't be adjusting the fuel pressure. We don't know how accurate my cheap pressure gauge is and at this point I don't think it matters. The pressure that worked best on a new car in the factory in 1972 might not be the pressure that works best on this particular car, with this mixture of old and new components, some original and some after-market. I see the 28 psi as a starting point for the tune, not necessarily the right end point.

Third, I will check the advance mechanism (weights and vacuum). Easy enough and worth doing. I'm skeptical of the benefits it will show, however, because of the way the timing is set for this engine (at 3,000 rpm). Clearly the centrifugal advance is working well enough for that, so the timing with the hoses connected shouldn't be far off, if it's off at all.

Again, thanks all. I'll report back with more results when I have them.