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Car starts than dies after 2-3 seconds. already replaced all…

Car starts than dies...

Car starts than dies after 2-3 seconds. already replaced all plugs coils and MAF sensor

Mechanic's Assistant: Does the engine stall right after starting? And what about hesitation or jerking during acceleration?

Cars runs fine with MAF unplugged

Mechanic's Assistant: What is the model/year of your Porsche?

1998 porsche boxster

Mechanic's Assistant: Are you fixing your Boxster yourself? What have you tried so far?

Removed MAF sensor and car starts and runs fine even with the sensor out of the intake tube and still hooked up to harness replaced all plugs and coils

Mechanic's Assistant: Anything else you want the mechanic to know before I connect you?

Maf unplugged or removed from intake tube the car runs and drives fine ok thank you

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Answered in 1 hour by:
3/23/2018
Matt
Matt, Engineer - BEng Hons
Category: Porsche
Satisfied Customers: 23,303
Experience: Mechanical Engineer with 20 years experience in the auto industry, 8 yrs in formula 1 engine testing
Verified

Hello and welcome to Just Answer

by disconnecting the MAF you will force the ECM to use other sensors to run the engine

So it would 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

also check that for 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 /propaneor 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.

1 vacuum leak path that you won't find with the above test is if the brake servo is leaking

so try clamping off servo hose to see if this has any effect

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Thanks Matt

Matt
Matt, Engineer - BEng Hons
Category: Porsche
Satisfied Customers: 23,303
Experience: Mechanical Engineer with 20 years experience in the auto industry, 8 yrs in formula 1 engine testing
Verified
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Customer reply replied 3 months ago
i have also ordered a new oil fill tube because i noticed a little oil on the top of the case but still has the same problem, right now i have the throttle body off and intake plenum off being cleaned and will reinstall those when i get home form work today
Customer reply replied 3 months ago
I have gone thru and checked every hose and clamp I could see and car still won’t stay running with the maf installed into the intake tube. But the car runs fine if the maf is plugged in and out of the tube

OK next step then is to check for exhaust leaks before the O2 sensor as if this has been experiencing an exhaust leak then it will have been running very rich as the sensor will see the extra air pulled in as fresh / lean fuelling and be adding fuel to compensate.

it cna only do this when it has a MAF input so even if the MAF is OK a faulty O2 sensor will steer you wrong

this can lead to catalyst failure as the extra fuel can re-light inside and melt the catalyst

The best way to find an exhaust leak is to run the engine from cold and feel around the exhaust joints with your bare hands - you'll have about 30 seconds before it all gets too hot and you should feel any escaping gas blowing over your fingers

also check the condition of the O2 sensor wires and if you just turn on the ignition without running the engine you should feel both of the primary O2 sensors ( the ones closest to the engine) get warm after about 1-2 mins

replace if they do not

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Customer reply replied 3 months ago
Ok I will check those today. Thank you.

understood

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