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Matt, Mechanical Engineer
Category: UK Car
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Experience:  BEng hons Mech engineering, in auto industry 22 years
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Toyota hiace: i have a 2003 toyota hiace 2.5 d4d,at times when

Resolved Question:

i have a 2003 toyota hiace 2.5 d4d,at times when i stop and try to re-go,the engine loses power but trys to rev up,leave it drop to idle again and it will be fine again,does not happen whilst driving,only from idle.No bad smoke and starts fine................
Submitted: 2 years ago.
Category: UK Car
Expert:  Matt replied 2 years ago.
Hello

This could be a lack of fuel pressure from the electrical pump at the tank.

If the connections to this 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 inlet to the main engine fuel pump 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.

If both of these are OK then also check the connections to the EGR valve, if either the vacuum pipe of electrical connection are damaged / corroded then the EGR can be on all time which certainly will hurt performance. Its also possible that its a fault with the EGR valve so its worth removing the valve and cleaning it out with brake cleaner, if it looks particularly clogged then replace it.

If this is OK then check the small vacuum lines to the turbo and its connected control solenoid on the bulkhead, any cracks or leaks can give turbo issues and its best to have the boost pressure measured to check that the turbo is healthy

If its a variable vane turbo (they’ll be an actuator on the turbo body - but not a wastegate) then check its vacuum pipework as above and check the connections to the diaphragm / solenoid valve


Also worth considering a bottle of injector cleaner into the tank as a clogged injector nozzle will reduce power and give poor combustion - the next stage on from this is to remove all the injectors and have them ultrasonically cleaned and flow checked

Also check all the high pressure hoses post turbo/s for any split hoses or cracked metal pipes, also check the ends of the intercooler as its not unheard of to pop the end caps off under high boost.


If the above are all OK then try checking for airleaks after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and leans the engine out causing rough running. As its a mechanical fault it tends not to turn on the fault light and you can sometime 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 around each joint in turn. If the engine rev's up you've found your leak.
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.

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

I'd also suggest inspecting the throttle pedal sensor for corroded contacts and damaged wiring

and finally its worth doing a compression test on all cylinders just to ensure the that the base engine is OK
Customer: replied 2 years ago.
Tanx, but how come when its left to drop to idle again,its suddenly fine and power back and could be for a week,seems to be something that clicks in when left to idle speed
Expert:  Matt replied 2 years ago.
Hello

in that case it sounds like an electrical fault
this could be a wiring issue as things tend to expand when hot and its possible a poor or partial electrical connection is being pulled apart on one of the major sensors so check the connection plug and wiring to the following;
Airflow meter
Crank position sensor
Cam position sensor
O2 sensor
Throttle pedal position sensor
Also worth checking the connections to the fuel pump for the same reason

and also unplug and inspect the main power relays for signs of overheating or corroded terminals - so the ones for the ECU and the fuel pump

And also unplug the wiring loom connections in the engine bay one at time and inspect for corroded or recessed pins.
Matt, Mechanical Engineer
Category: UK Car
Satisfied Customers: 16596
Experience: BEng hons Mech engineering, in auto industry 22 years
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