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Matt
Matt, Engineer
Category: Buick
Satisfied Customers: 21484
Experience:  Honors degree in Mechanical Engineering, worked 8 years as a Formula 1 engine engineer.
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I have a 95 Le Sabre that misses whenever engine manifold

Customer Question

I have a 95 Le Sabre that misses whenever engine manifold vacuum drops below 5 inches. A scan tool shows various cylinders misfire and shows no other codes. System numbers and voltages are in range for normal values, I am using an Actron scan tool. The engine runs great above 5 inches of vacuum.
Submitted: 1 year ago.
Category: Buick
Expert:  Matt replied 1 year ago.

Hello

this sounds like the issue is when the ECU is in closed loop control ?

so I'd suggest to 1st 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.

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 over the state of the O2 sensor wiring and connector and also try disconnecting it to see if this changes or improves the situation

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 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.

Expert:  Matt replied 1 year ago.

Hi

do you still need help?

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