My Porsche idles at the redline

Question:

I own a 1971 1.7-liter 914 Porsche, which has the factory fuel injection system. A few years ago I changed all of the vacuum lines, one by one. I then started the engine which proceeded to rev through the roof so I quickly shut it down. I double-checked the vacuum lines with the shop manual and they appeared correct. I checked the timing, which seemed OK.

After much fiddling and frustration with no success I took the car to a “type IV engine specialist.” He checked that the vacuum lines were correctly installed and they were. He replaced the worn intake plenum hoses, replaced the plugs, and checked the timing but it still revved to the redline on start-up. By the way, it ran fine before I replaced the old vacuum hoses.

The mechanic suggested that I remove the fuel injection system and install carburetors. I said “no thanks” and $500 later brought the car home.

I’d like to get this car on the road so I can sell it for what it’s worth and put the money toward restoring my 1958 Apache pickup.

Answer:

The high rpm condition you are experiencing is usually due to one of two maladies. The first is a mechanical malfunction with the throttle system and the second is a bad vacuum leak.

Visually inspect every inch of the throttle system from the accelerator pedal all of the way to the throttle plate to determine if the system is hung up or otherwise messed up somewhere.

The best starting point for diagnosing a vacuum leak is with a vacuum gauge. But, of course, that’s useful only if you can run the engine. If you really can’t get it to run at something less than the redline, try looking for vacuum leaks with your eyes. The Bosch D-Jetronic electronic fuel injection system happens to be prone to vacuum leaks and unlike the subsequent L-Jetronic system it does not run lean when suffering from a vacuum leak. Instead, engine rpm simply climbs in proportion to the severity of the leak.

Though you have already replaced the hoses and triple-checked that they are correctly installed, they bear one more examination since the problem arose immediately after you installed them. Make sure that each hose is in sound condition, with no splits or other defects.

Other common sources of vacuum leaks with the D-Jetronic system include intake manifold gaskets, injector seals, intake runner boots, intake manifold plenum (look for cracks along seams or around fittings), throttle body seal and shaft, auxiliary air regulator, deceleration valve, cracked distributor advance or retard diaphragm, stuck open PCV valve, cold start valve (either the actual valve or its seal to the intake plenum), or manifold pressure sensor.