850 Cars: Engine rpm when changing gears

[klausc]

Hello gents. I usually just read a lot on your wonderful site, and learn a lot. Unfortunately, we do not have very many manual transmissions in the USA and this interesting item came up on the Brickboard.com:

When shifting gears, the 2.4L 20Valve engine increases rpms for a second or two before responding to the throttle being closed. Is this normal? Does this happen because of our smog related hardware? The rpms only increase about 50 or so, not a real big deal but still frustrating.

It is still too cold for me to work in my garage, otherwise my throttle body would be off and on the workbench. If this is known to be a computer generated 'problem', the few of us that have manual transmissions will stop looking for the non-existant problem.

Thank you so much,

Klausc

[warninglight]

My first guess would be sticking throttle cable, but if yours has the later ETM, that rules out that option!

I've never experienced a manual car that displays those symptoms, apart from having a sticky throttle cable in a classic car. Can't see any reason for it to be programmed in, especially when you consider clutch wear, and how smooth the cars generally are.

[scutyde]

Check for air leakeage in the intake. This is how my hose from the air cleaner to the intake manifold looked like: http://www.motor-talk.de/bilder/fall...205950395.html

Also look at the small vacuum hoses they tend to break after all these years.

Cheers
Cristian

[klausc]

Cristian, that is probably THE worst intake hose failure I have ever seen! Yes, I intend to replace all of the vacuum lines shortly.
The Throttle cable is not sticky. As soon as my garage temperature gets above 10C, I will begin tearing down and cleaning the throttle body and disabling the airbox thermostat so that I will only get cold air.

I am just curious regarding the 50rpm jump when up shifting, as others in the USA have reported the same thing. Perhaps it is some code in our pollution control mechanism, ECM. If it is vacuum or mechanical related, I expect to fix it, but if it is inside the engine computer...

[Tension]

Originally Posted by klausc

Cristian, that is probably THE worst intake hose failure I have ever seen! Yes, I intend to replace all of the vacuum lines shortly.
The Throttle cable is not sticky. As soon as my garage temperature gets above 10C, I will begin tearing down and cleaning the throttle body and disabling the airbox thermostat so that I will only get cold air.

I am just curious regarding the 50rpm jump when up shifting, as others in the USA have reported the same thing. Perhaps it is some code in our pollution control mechanism, ECM. If it is vacuum or mechanical related, I expect to fix it, but if it is inside the engine computer...

The 850 likes a bit of warm air :/

[DaveT5]

When you clean out the throttle body, I would also do the idle control valve. I found this considerably improved mine (was sometimes increasing by 200rpm on gear changes).

[klausc]

This is from another American's post on the Brickboard with the same problem but better details:


I have been diagnosing an intermittent backfire problem with a recently purchased 1998 V70 NA manual trans car. Sometimes a backfire occurs when shifting. Klaus asked if the engine RPM momentarily increases during the shift intervals when the clutch is disengaged and the throttle is released. The RPMs do increase, unlike any other manual trans vehicle I have ever owned.

The throttle body, IAC valve and flame trap hoses are clean. No vacuum leaks that I can find. The car starts, idles and drives smoothly. No OBD2-DTC codes.

I have isolated the shift interval momentary RPM increase to the IAC system. With the transmission in neutral, slightly pressing the throttle increases RPMs to 1000. A little more throttle and the RPMs go to 1200, then quickly surge to 2000-2200 RPM without additional throttle input.

Blocking the IAC inlet hose with a solid plug eliminates the off-idle surge RPM increase. With the IAC inlet blocked, the neutral/off-idle RPM can be held at any value from 1000 RPM to 2000 RPM with no surge. Also the momentary RPM increase during the shift intervals is eliminated.

So the IAC system is causing the problem, but why ? The IAC valve is clean and the internal vane rotates smoothly. The IAC valve control signals (checked with an oscilloscope) are correct. A replacement IAC valve behaves the same.

I finally realized that the ECU is commanding the IAC to open up at off-idle RPMs > 1200. This was confirmed with a digital voltmeter that averages the IAC Pulse Width Modulated "valve open" signal.

My OBD2 scan setup can display multiple real-time parameters such as RPM, absolute throttle position, O2 sensor voltage, ignition advance, etc. The reported throttle position is stable 10.5% at idle and about 85% wide open. In neutral, an 11.5%-12% throttle position increases engine speed to 1200 RPM and initiates the surge effect. I am beginning to wonder if the ECU firmware is incorrect. I havent checked to see if the ECU part number is correct for this manual trans car.

The other reference I have is my daughters 1998 XC70, but it has turbo and auto trans. It does not exhibit the neutral/off-idle surge effect described above.

I expected the IAC system to be inactive if the engine is off-idle, that is not the case with this car. I dont understand how the ECU can accurately determine an off-idle condition from the throttle position sensor. The TPS reports 10.5% with the throttle closed. In neutral a 12.5% throttle position is enough to produce 2000 RPM engine speed.

Any suggestions are appreciated. Next I plan to check the ECU version and perhaps try a new TPS.

[Shortstuff]

Before you buy a new TPS try cleaning it and the Idle Control Valve with brake cleaner. It's worth a try and doesn't cost half as much as new parts

I had similar ages ago but it set the MIL off and turned out to be a vacuum leak on the air intake. I'm not sure if that would be your problem tho since I don't know if your engine works on the same vacuum system as mine but it could be worth looking at