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Matt, Mechanical Engineer
Category: UK Car
Satisfied Customers: 21869
Experience:  BEng hons Mech engineering, in auto industry 22 years
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My friend has a 55 plate Chevrolet tacuma 1.6 the engine

Customer Question

My friend has a 55 plate Chevrolet tacuma 1.6 the engine idle is lumpy and when you are accelerating and changing gear the revs stay up and only drop slowly can you help
Submitted: 2 years ago.
Category: UK Car
Expert:  Matt replied 2 years ago.


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.

Customer: replied 2 years ago.
Hi our local garage has ruled out a vac leak.he thought it might be the stepper motor.since then I have fitted another throttle body.what puzzles me is is you sit and just increase the revs to say 2500 /3000 hold it for a few seconds then release the revs drop straight back to idle.without hesitation?
Expert:  Matt replied 2 years ago.
thanks for the extra information I'd say that this could be an issue with oil breather system
The first thing I'd check is the crankcase pressure relief valve (PCV) these are sprung loaded and are only meant to 'burp' excess pressure every now and then. The exiting gas is piped back to the inlet, however should the valve stick open then the engine tends to suck the air out of the crankcase all the time and this carries oil over with it.
If you remove the air intake pipe and look inside where the crankcase breather is plumbed in you may find evidence of oil.
If you do find oil replace the valve
Customer: replied 2 years ago.
Ok can you tell me where I will find this on my engine
Expert:  Matt replied 2 years ago.


the PCV is usually a screw in valve on the cam cover or it may be inside the rubber breather pipe that runs from the cam cover to the enigne inlet

trace the breather pipe 1st and it will lead you to the PCV

Customer: replied 2 years ago.
Thanks matt as soon as I get a chance to look ill let you know how I get on
Expert:  Matt replied 2 years ago.
no problem
just get back to me on this post
Customer: replied 2 years ago.
Hi matt sorry none of the info helped thanks for trying
Expert:  Matt replied 2 years ago.
this sounds like a lack of fuel and could be a lack of fuel pressure from the electrical pump at the tank.
If the connections to the pump are corroded or damaged then the pump could stop at any time or could not be running at full speed. Check that its relay switches in and out and the relay contacts are clean and bright – replace the relay if in any doubt and do the same for the fuse,
Ideally measure the pressure at the injectors and if this is low check the pump as described and also consider changing the fuel filter as if this partially blocked this too will reduce pressure.
Expert:  Matt replied 2 years ago.
do you still need help?