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Oscillating idle

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Old Jul 22nd, 2012, 08:54   #1
pistrix
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Default Oscillating idle

Hi Everyone,

after a month of driving on LPG (1995 Volvo 850 20V - Prins) I still have a problem that when the engine is cold (but not immediately after it starts to run on LPG) the idle is very rough. It oscillates with about 1Hz below the normal idle RPM. I have no error codes and no warning lights. When running on petrol, idle is frozen still. I replaced the spark plugs, wires, distributor cap and rotor, cleaned the throttle body and idle control valve.

Any ideas what could be wrong?

btw, last time when they tuned the Prins system, they told me that the Lambda needs quite some time to "wake up" and starts responding with the proper oscillations...

Thanks,
Jan
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Old Jul 22nd, 2012, 22:00   #2
classicswede
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Where are the manifold jets positioned in teh manifold?
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Old Jul 23rd, 2012, 15:01   #3
pistrix
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Default manifold jets

Just by the petrol injectors (approximately 0.5 - 1.5 cm in the direction away from the engine)
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Old Jul 24th, 2012, 13:01   #4
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Default very cold

One thing I would like to add: These oscillations occur only when the engine is really cold like in the mornings. There are no oscillations during the day, even when the car is not running for 8 hours when I am at work and it still takes some time to switch from petrol to lpg...

Strange!
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Old Jul 24th, 2012, 20:14   #5
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If the lambda is slow to start working it is most likely due to the heater not working. The ECU does not start trimming the fuel untill the lambda has warmed up and working.

A common mistake Prins installers make is not T'ing the water connections in but putting the reducer in line. How are the water connections made?
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Old Jul 25th, 2012, 10:17   #6
pistrix
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Default T and lambda

Quote:
Originally Posted by classicswede View Post
If the lambda is slow to start working it is most likely due to the heater not working. The ECU does not start trimming the fuel untill the lambda has warmed up and working.
Do you think this might be the reason? Should I replace the lambda? Is the heater integrated in the lambda sensor?

Quote:
Originally Posted by classicswede View Post
A common mistake Prins installers make is not T'ing the water connections in but putting the reducer in line. How are the water connections made?
I guess they used T parts to connect the water. Image: https://www.dropbox.com/s/75sa98y0b3sxg5s/IMG_0643.JPG Is this ok?

Thank you very much for your help!
Jan
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Old Jul 25th, 2012, 20:41   #7
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I doubt the lambda is the issue with the idle

The water T's are done cerrectly so that will not be the issue either.

I think the only way of finding out would be to moniter the fuel trims.
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Old Oct 15th, 2012, 12:50   #8
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Default Solved

Hi Everyone,

I have solved the problem with oscillating idle when the engine was cold. The fault was in incorrect readings of water temperature sensor. When I replaced the sensor with a new one, the problem went away immediately.

My logic behind:
Incorrect temperature information (at low temperatures) caused incorrect petrol injection. Because the Prins system imitates the petrol injection, the gas/air mix was incorrect. Further, because the sensitivity of the engine to the gas is much higher than petrol, oscillating problems occurred when driving on gas but not when driving on petrol.
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Old Nov 14th, 2012, 08:58   #9
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Default NOT solved

Hi again.

the problem with my oscillating idle when cold came back. After replacing the water temperature sensor, the problem significantly improved right after the switch between petrol and gas but I have now oscillating idle later when the engine is almost warm and the temperature gauge is already at 3PM. It then goes away in a few minutes but I guess it will take longer when the winter comes.

The Prins guys tested the car for two days and say there is nothing to do and something must be wrong with the engine. The Volvo guys didn't even want to check saying if everything works on petrol, nothing is to be checked... They just gave up - my 850 is terminally ill ). But I haven't (can't) and now I am studying the principles of how the computer controls the injection and how the sensors influence the injection... I thoroughly and carefully cleaned the MAF sensor yesterday which also gives the information about the air temperature but the problem still persists. Oxygen sensor is now the only sensor that I haven't either cleaned or replaced. It is still the original one (17 years old). I will try to clean it.

Next, I will blow warm air in the air intake and see if this helps. If it does, than MAF sensor probably needs to be replaced. If the problem won't go away, I will have to think of a new strategy. Maybe the ignition coil got weak...

I would very much appreciate any ideas what to do next...
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Old Nov 15th, 2012, 12:34   #10
capt jack
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Default

Hi there

I have exactly the same problem with my 1997 V70 LPG conversion - oscillating idle when cold. On petrol it is running absolutely perfectly.

The LPG system is a Tartarini system, and has been on the car for 8 years and covered over 220,000 miles. In total the car has done almost 280,000 miles.

I've fitted new LPG injector solenoids, a new LPG filter, and have replaced the thermostat and the coolant temperature sensor. I also thoroughly cleaned the throttle body and fitted new spark plugs, and had the fuel trims re-mapped. All these things helped a bit, but none of them completely cured the problem.

Having spoken with the fitting company, they say that the most likely cause is that the gas regulator is simply worn out, and that it needs to be replaced. The rationale being that the system cannot maintain gas pressure when cold. Tartarini do offer a service kit for the regulator, but I reckon that as the regulator has done such a large mileage, it would be more sense to replace it.

I am going to replace the regulator and the gas temperature sensor next week, so I will post the outcome on here.

The main worry is that although the car drives well on petrol, and reasonably OK on LPG when hot if I run it on LPG for any length of time the LPG ECU upsets the petrol ECU, then I need to re-set the petrol ECU by disconnecting battery, or the car won't start on petrol! I also get random fault codes, all related to fuel trim. If I don't use LPG at all then car runs very well indeed, with no oscillating idle, no difficult starting and no fault codes.

I am hoping that a new regulator and a fresh re-map does cure the problem because petrol is getting expensive!

Cheers

Jack

Last edited by capt jack; Nov 15th, 2012 at 12:38.
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