97 US 850 non turbo starting question

What is the mileage? Are there any codes present? When were the ignition parts last replaced? When the temperature drops the engine coolant temp sensor signals the fuel side of the computer to increase the injector(s) pulse width and enrichen the mixture. A slightly enrichened mixture will fire more easily in the presence of a spark. An over rich mixture will foul the plugs, which is what happened.

This can be caused by a number of things. Coolant temp increased resistance: shrinkage at a joint or connector can move the contact surfaces of the connection to a less conductive area of contact. Usual sources are computer grounds, circuit grounds, high resistance solder joints on circuit boards or decreased signal strength from the cam sensor, engine speed sensor or the mass air flow sensor.

Additional possibilities include a seeping fuel pressure regulator, cracks in the inlet hoses or an intake manifold leak of some sort allowing unmetered air into the motor.

Multiprong spark plugs were recommended to alleviate the problem in non turbo cars, along with a plastic spacer that prevented the timing belt from jumping on cold starts after an extreme temperature drop. One of the more interesting properties of rubber is that it tends to shrink when hot and expand when cold. Consequently the belt tensionser has to respond in a parallel manner in order to keep the belt tension relatively constant. Unfortunately the tendency of the tensioner to stick when cold and old will allow the belt to jump unless the spacer has been installed to prevent jumping. At any rate, the slight retardation of cam timing due to belt growth can contibute to hard start problems if the ignition components are marginal or well worn.

An additonal consideration is that continued rich hard starts will contaminated the oil with fuel and exacerbate the starting problem as well as allow increased internal engine wear from decreased lubrication.

If you can't check these things have them checked by someone who is familiar with the car.

Bob

I've noticed that too, in both my 98s. They seems to "learn" exactly how much fuel is needed on cold start, and swings of more than 10-15°C seems to confuse them.

125000 miles

Are there any codes present?

The check engine light is not on--only time that went on was when my wife did not 'click' the gas cap.

When were the ignition

It had the 120,000 check 5k ago. I do not know exactly what they did do--but it has always worked fine, the quick temp change seems to be the triggering event.

When the temperature drops the engine coolant temp

These sound possable--the sudden change in temp has caused a PC board with differing components to contract at different rates...

These would seem to be faults that would continue to cause trouble. The problem only seems to occur when there is a sudden drop in ambiant temp. In the instant case the last time the car operated it was warmish out in the

60's F the next cycle, when the cold start problem occured was at a temp of about 29F.

After a run of 30 minutes or so and a cold soak of about four hours in the

29 or so F weather the car started fine and ran well. I have not run the car since this issue arose.

Thanks for the ideas--I will have the car checked out if the problem re-occurs. How does the 850 sense ambient temps? Clearly less fuel is required for a warm outside air temp start of a cold engine then for a cold outside air temp start of a cold start.

If the component that senses the OAT is a bimetallic device perhaps in time it gets tired and will react to the changes in outside temp much less in time...

Additional info--

The temps have been between 20 and 24 F and the car has cold started from 8 hr or longer cold soaks flawlessly.

The problem is just limited to the behavior of the car on the FIRST start after a sudden temp drop.

Any ideas, Please!

The primary source of temperature information comes from the engine coolant temperature sensor. This is an NTC device. The internal resistance decreases as the coolant temperature increases. The ambient air temperature is determined by a thermistor in the center of the intake air stream at the Mass Airflow Sensor. Both bits of informaton are available through the OBDII Data Link Connector to either the Volvo system tester, VADIS/iVADIS or the Autodiagnos scan tools. When scanning the car cold, after a long cold soak, the thermometer on the wall, the ambient air temp sensor behind the spoiler and the two engine sensors should all read within a few degrees of each other. In terms of failure, the ECT sensor has been more problematic than the foil type MAF sensors. However, both would trigger the MIL if the fault persisted for more than several seconds.

If when the scan tool connected the readings seem true then the next step would be to inspect the vacuum line at the fuel pressure regulator for the presence of fuel. None should ever be there. Intermittent diaphragm leaks on 850/70 cars are particularly hard to catch and don't fail often.

Failing that there is a connector or ground problem problem that will have to be addressed by physically inspecting each one and eliminating each as a cause until the only thing left is the control unit. The ECUs are not notoriously weak points, but they hold the highest probability for defective wave soldered joints.

If the signals for the engine speed sensor or the cam position sensor were absent, neither would set a code unless the engine were running and if either were absent there would be no injection. The overrich problem was quite prevalent in the turbo models and a service kit was provided as a bulletin fix for them. There may be an ECU reflash available for your car. There is an updated ECU, most updates address cold start fuel mapping issues.

Bob