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Matt
Matt, Mechanical Engineer
Category: UK Car
Satisfied Customers: 20591
Experience:  BEng hons Mech engineering, in auto industry 22 years
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The car works perfectly in IDLE but as soon as I press the

Customer Question

the car works perfectly in IDLE but as soon as I press the GAS pedal the engine starts waving from low to high rpm and this goes on until i release the gas, then the engine remain correctly in idle (see the attached video).
JA: How many miles are on the car? What size is the engine?
Customer: 120000 miles B5202S Engine
JA: Are you hoping to fix this yourself? What have you tried so far?
Customer: - Ignition looks good, i have checked wirings, spark plugs... everything looks fine
- FPR not tested yet
- Fuel pressur I have tested 1 week ago: 35 PSI
- TPS: tested looks ok
- MAP sensor and MAP hose look good
JA: Anything else you think the mechanic should know?
Customer: no
JA: OK. Got it. I'm sending you to a secure page on JustAnswer so you can place the $5 fully-refundable deposit now. While you're filling out that form, I'll tell the Volvo Mechanic about your situation and then connect you two.
Submitted: 5 months ago.
Category: UK Car
Customer: replied 5 months ago.
I have a Volvo 850 that i bought last year. The car is 1995 Engine B5202S equipped with Fenix 5.2 ECU.Currently the car works perfectly in IDLE but as soon as I press the GAS pedal the engine starts waving from low to high rpm and this goes on until i release the gas, then the engine remain correctly in idle.HISTORY OF THE CAR PROBLEMSI bought the Car in 2015.I have had at the beginning a random no start condition, by following instruction on youtube I have succesfully replaced the CMP sensor and everything went well, until few months ago when the car was not starting again. I have found, again following instruction on youtube, that there was a Crankshaft sensor bad connection, then i repaired the connection and the car started immediately well.BUT, and this is something that is driving me crazy, the car works perfectly in IDLE but as soon as I press the GAS pedal the engine starts waving from low to high rpm and this goes on until i release the gas, then the engine remain correctly in idleI m an electronic engineer so i decided to make something like a breakout box. I had bypassed all the 86 pins of the ECU and then i had now the possiblity to test almost all the wiring and also the signal from/to ECU. I also have an oscilloscope that help me (see picture attached here: https://drive.google.com/file/d/0B-vFeitjfWYwMklQYzU4UXpPT0U/view.)I have also made the circuit to read OBD codes: the only error code that i have is 1-2-2 that is the IAT sensor. BUT i have to say that even when the car was working properly, the 1-2-2 code was set. So it cannot be the cause of the issue. Testing the input in the ECU i have found this is set to 4,5 volt... to me it looks like someone made a modification to tell the ECU that the sensor read always fresh air so as to have a bit more rich fuel mix... i have heard that some people was doing that... so, since the car is 4th hand maybe this is the case.Anyway I am quite sure this is not the problem.I have tested the Throttle sensor and is ok and the MAP sensor and it's ok.Then i have looked to the Injectors signal, (i have attached the video of the ocilloscope for your review: https://drive.google.com/file/d/0B-vFeitjfWYwaVJPUnZNbFNlc00/view). You can clearly hear the engine sound when i open the gas and look at the signal.
Customer: replied 5 months ago.
The right links to the pic and the videohttps://drive.google.com/file/d/0B-vFeitjfWYwMklQYzU4UXpPT0U/viewhttps://drive.google.com/file/d/0B-vFeitjfWYwaVJPUnZNbFNlc00/view
Expert:  Matt replied 5 months ago.

Hello

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 flattenedpiece 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.

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

Air leaks are very temperature dependent as gaps can open or close up as things expand with heat, so the weather and engine temperature can effect them.

This leads them to be quite intermittent in the case of mild leaks

its also worth cleaning the airflow meter by removing it from the car and spraying the exposed sensor wires inside the tube with a brake or switch cleaner - ensure that the cleaner is one of the old fashioned, non Eco type that does not leave a residue

On no account touch the sensor wires with anything physical as they are extremely fragile

Expert:  Matt replied 5 months ago.

Hi

do you still need help?

Bear in mind that the site takes a deposit from you at the beginning and this is held by the site until you rate my answer at which point the cash is split between the site and the expert. I am only paid for my work on this question if you rate my answer, using the star system at the top of the screen. Please do not forget! Thank you

Customer: replied 5 months ago.
Dear Matt thanks so much for the answer. Tomorrow morning i will work on the car and will try your suggestions. I will let you know if problem will be solved and of course i will rate your answer or ask for mor help if needed. Thank you !
Expert:  Matt replied 5 months ago.

understood

good luck

Customer: replied 5 months ago.
Hello Matt, I still have the proble, i did the test you suggested about the propane lighter and found NO leakage, except a very very very very small engine revs up (again very small) around the fuel injectors manifold, close to the fuel pressur regulator. Then i immediatley thought: "Thanks Matt, problem found!". I then verified the hose and pipe but it was ok, then I dismounted all the fuel injectors manifold to see if the pressure regulator was bad or leaking (because in this car the FPR is located underneath the fuel injectors manifold) but found nothing. Then i mounted again everything in position and connected a pressure meter to the fuel manifold and did some test. In IDLE fuel pressure is about 35 PS. If you disconnect the FPR vauum the pressure rises to 45 PSI. I also tested using a syringe the pressure variation of the injectors manifold and you can see everything in the following video. Listen to the the engine sound when open the throttle and look to the test I did.
Here is the video: https://drive.google.com/file/d/0B-vFeitjfWYwa1YwMTN6bEZVRjg/view?usp=sharingI also checked again the TPS is ok reading from .45 to 4.5 volts and also tested the MAP sensor and it reads OK.
What i still didn't test is the Idle Air control Valve but i don't think it can create such issue.
Please i REALLY NEED the car working so any suggestion is welcome!
Expert:  Matt replied 5 months ago.

Ok

I'd next remove the air intake so you can see the throttle and with the ignition on but engine off then have your assistant slowly push down on the gas pedal whilst you watch the throttle butterfly

it should move smoothly and slowly with no jerks

if its sticking then try cleaning it out and if its still sticking then replace it

Expert:  Matt replied 5 months ago.

Hi

do you still need help?

Bear in mind that the site takes a deposit from you at the beginning and this is held by the site until you rate my answer at which point the cash is split between the site and the expert. I am only paid for my work on this question if you rate my answer, using the star system at the top of the screen. Please do not forget! Thank you

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