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Matt, Engineer - BEng Hons
Category: Porsche
Satisfied Customers: 21991
Experience:  Mechanical Engineer with 20 years experience in the auto industry, 8 yrs in formula 1 engine testing
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I have a 1990 C4 Cab (125,000 miles) that intermittently

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I have a 1990 C4 Cab (125,000 miles) that intermittently hesitates & lurches. There is no change in the engine sound & it doesn't stall, but it feels like the power has been disconnected for a brief interval. None of the mechanics have been able to diagnose a fault, including a usually reliable dealer service dept. Any thoughts?

This could be an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.
As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane or brake cleaner around each joint in turn. If the engine rev's up you've found your leak.
Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.
Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

Air leaks are very temperature dependent as gaps can open up as things expand with temperature so depending on how the leak occurs ( on a hose joint with a clamp expansion will help seal and on a vacuum hose expansion ill make a leak worse) so leaks can be better or worse as the engine heats up.

It’s also worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a poorly pump will all result in insufficient fuel being delivered to the engine

Might also be worth checking the wiring and connector to the airflow meter for any signs of corrosion or damage. you can do a quick fault find if you unplug the meter and run the engine without it.

if the engine condition is the same then chances are the meter or the connection to it is faulty

It’s also worth cleaning the MAF sensor wires, as they can get coated with dirt over time which then offsets the reading
Use some contact cleaner or brake cleaner to spray onto the wires to remove the dirt – on no account touch the wires with anything as they are very fragile

Customer: replied 2 months ago.
The engine is not running rough or revving. As I said, there is no indication of misfire. The problem is strictly the feel (not the sound) of interrupted power.


well a mild / small airleak wouldn't produce a misfire you'd just get this slight hesitation that you're describing, also airleaks are often very cheap to fix usually being a hose clamp not tight enough or a split hose. So I'd still say its worth doing the examination next

Customer: replied 2 months ago.
Dealer mechanic advises he has done everything you suggested & still hasn't solved the problem.



another thing to try is to clean out the throttle body as they can get clogged up and this hurts the throttle response

also if the fuel pressure hasn't been checked then this also needs doing

if thats OK then I'd consider replacing the spark plugs with new items

and does running on high octane 93 help?

Customer: replied 2 months ago.
The fuel pressure was checked. I always use high octane gas. The spark plugs were replaced not long ago. I'll ask about the throttle body.


and one other thing that occurs to me when was the air filter last changed?

Customer: replied 2 months ago.
I don't remember, but will check.


just get back to me on this post

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