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Engine RPM waves up and down by pressing gas pedal

Help, Advice and DIY Tutorials on Volvo's extremely popular car line -- Volvo's 1990s "bread and butter" cars -- powered by the ubiquitous and durable Volvo inline 5-cylinder engine.

1992 - 1997 850, 850 R, 850 T5-R, 850 T5, 850 GLT
1997 - 2000 S70, S70 AWD
1997 - 2000 V70, V70 AWD
1997 - 2000 V70-XC
1997 - 2004 C70

Captain_Hook
Posts: 4
Joined: Mon Jun 27, 2016 8:32 am
Year and Model: 1995 850 2.0 10v
Location: Italy
Has thanked: 0
Been thanked: 0
Italy
Captain_Hook

Engine RPM waves up and down by pressing gas pedal

Post by Captain_Hook » Mon Jun 27, 2016 8:44 am

Hello everybody, i am new to the forum and have a enormous problem.

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 PROBLEMS

I bought the Car in 2015.

I have had at the beginning a random no start condition, by following instruction on youtube provided by Robert DiY 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 idle (see the attached video below).

I 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-vFei ... pPT0U/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-vFei ... Nlc00/view). You can clearly hear the engine sound when i open the gas and look at the signal.

Unfortunately the car is about 100 miles far from my home so i can only work on it during the weekends.
Next saurday or sunday i can do more test on the car and send them to you if you will be willing to help.

Please, HELP ME! The car is quite old and in the little town were it is, is almost impossible to find people who knows that engine... i really do hope that someone here can help.

Thanks a lot to everybody and hello from Italy
Volvo 850 1995
2.0 10v Manual Transmission
Engine B5202S ECU Siemens Fenix 5.2

Sommerfeldt
Posts: 1136
Joined: Tue Jul 29, 2008 1:25 pm
Year and Model: 1996 850 T5
Location: Oslo Area, Norway
Has thanked: 55 times
Been thanked: 36 times
Norway
Sommerfeldt

Re: Engine RPM waves up and down by pressing gas pedal

Post by Sommerfeldt » Tue Jun 28, 2016 3:41 am

Yikes...

First of all, I'd say get all the junk that isn't OEM out of the engine bay. Get back to basics. See if you can clear that DTC for the IAT - i.e. replace that sensor if it's indeed faulty. Before making breakout boxes and whatnot, make actual use of what the car is telling you.

Second, is it up to Stage 0? How's your ignition looking? Vacuum hoses? FPR? What's your fuel pressure? Are your plugs gapped correctly? With these engines, you need to go through the motions - anything else is bound to cost you in the long run.

A modification to the IAT signal sounds like a pretty extreme way to go for a 10V 2 liter to me - what would be the purpose?

- S
'96 855 T5, R bumper and spoiler, Koni Yellows & blue H&R springs all 'round.

Captain_Hook
Posts: 4
Joined: Mon Jun 27, 2016 8:32 am
Year and Model: 1995 850 2.0 10v
Location: Italy
Has thanked: 0
Been thanked: 0
Italy
Captain_Hook

Re: Engine RPM waves up and down by pressing gas pedal

Post by Captain_Hook » Tue Jun 28, 2016 4:43 am

Sommerfeldt thanks a lot for your answer. Tomorrow i will have the possibility to check again and i will try to understand what the previous owners did about the IAT sensor and possibly to get back the wirings to stage 0. You are right about Stage 0... that's alway the best one can do for a car.

I will try to fix it before everithing else, even if i am quite sure that this is not the problem because i have always had the 1-2-2 code.

About you other questions:

- Ignition looks good, i have checked wirings, spark plugs... everything looks fine
- FPR not tested yet, i will do that and tell you the result
- Fuel pressur I have tested 1 week ago: 35 PSI
- TPS: tested looks ok
- MAP sensor and MAP tube look good

Vacuum hoses... what do you mean, sorry but since i m Italian my english is not so good

As for the modification of the IAT that some of th previous owner did.... i have heard that someone was usual to do that to give a more rich fuel mix to the engine... i would never to that... it is only an hypotesys.

I'll let you now!
Volvo 850 1995
2.0 10v Manual Transmission
Engine B5202S ECU Siemens Fenix 5.2

Sommerfeldt
Posts: 1136
Joined: Tue Jul 29, 2008 1:25 pm
Year and Model: 1996 850 T5
Location: Oslo Area, Norway
Has thanked: 55 times
Been thanked: 36 times
Norway
Sommerfeldt

Re: Engine RPM waves up and down by pressing gas pedal

Post by Sommerfeldt » Tue Jun 28, 2016 5:59 am

35 psi sounds low to me - I've got limited experience with the 10V's, but that seems almost 10 psi too low... I think. If you've got a gauge connected, you can pinch the return line and see if pressure rises rapidly (don't let it get too high).

Vacuum lines control seemingly everything on these Volvos, and they need to be in good condition. If the engine's taking in unmetered air, or if an FPR diaphragm is leaking fuel into the vacuum system, for example, symptoms are varied and confusing.

Modifying mixture like that might have some effect on a turbo... under boost. But I don't understand what purpose it would serve on a 10V 2.0 NA.

- S
'96 855 T5, R bumper and spoiler, Koni Yellows & blue H&R springs all 'round.

Captain_Hook
Posts: 4
Joined: Mon Jun 27, 2016 8:32 am
Year and Model: 1995 850 2.0 10v
Location: Italy
Has thanked: 0
Been thanked: 0
Italy
Captain_Hook

Re: Engine RPM waves up and down by pressing gas pedal

Post by Captain_Hook » Tue Jun 28, 2016 6:50 am

Thanks Sommerfeldt... if pressure rises rapidly that means that FPR is OK, right?
Volvo 850 1995
2.0 10v Manual Transmission
Engine B5202S ECU Siemens Fenix 5.2

Sommerfeldt
Posts: 1136
Joined: Tue Jul 29, 2008 1:25 pm
Year and Model: 1996 850 T5
Location: Oslo Area, Norway
Has thanked: 55 times
Been thanked: 36 times
Norway
Sommerfeldt

Re: Engine RPM waves up and down by pressing gas pedal

Post by Sommerfeldt » Tue Jun 28, 2016 7:05 am

Yeah, that means pump and all the other doodads are fine. :) But take the vacuum hose off the FPR and smell it before you do any of that - if it smells STRONGLY of gas, then there's a problem.

- S
'96 855 T5, R bumper and spoiler, Koni Yellows & blue H&R springs all 'round.

Captain_Hook
Posts: 4
Joined: Mon Jun 27, 2016 8:32 am
Year and Model: 1995 850 2.0 10v
Location: Italy
Has thanked: 0
Been thanked: 0
Italy
Captain_Hook

Re: Engine RPM waves up and down by pressing gas pedal

Post by Captain_Hook » Tue Jun 28, 2016 7:19 am

I will do that! Thanks so much... tomorrow morning, in a small village in nort west of Italy, i will do the test suggested by a Norwegian friend :D
I will let you know!
Volvo 850 1995
2.0 10v Manual Transmission
Engine B5202S ECU Siemens Fenix 5.2

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