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Discussion Starter · #1 ·
Hello,

I recently bought a 1986 Volvo 240 (245) DL wagon with 199K miles on it. It came with a surging problem, where the engine RPM oscillated when it was first started, but then the engine ran more uniformly once it was warm. When the car is first started and this surging is at its worst, the engine slows, then there is a ping and then the engine instantly picks up speed. Then it starts to slow and the process repeats. When I unplug a vacuum hose connected to the intake manifold to feel the vacuum, I can tell that the vacuum builds as the engine slows (more and more negative pressure), then when that ping happens there is positive pressure for a moment, then building vacuum again.

I read a few related articles on this web site and so I have been working on it.

First I cleaned the throttle body, which was dirty, but this did not help. Cleaning the idling motor was also recommended, but I could not find a part by this name in the manual. I did clean the adjustable valve connected to the throttling body.

Later I found a leaking vacuum hose leading from the intake manifold to a vacuum chamber in the center console. Patching this fixed the surging, but then the car ran very poorly. It just ran rough and seemed to run rich. The exhaust was black and putrid.

Next I pulled out and cleaned the PCV valves and hoses, etc. I found that the previous owner had disconnected the flame guard from the engine (underneath the intake manifold) After cleaning this out and hooking everything up, the car surged badly again!

If I unplug the mass air flow sensor, the car starts beautifully with no surging, but unless I depress the accelerator very slowly, the car stalls. It is not driveable this way, but this is what I do at the moment to warm up the car.

It makes no difference whether or not the oxygen sensor is unplugged.

I would appreciate any advice.
 

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The idle motor is under the intake with 2 hoses going to it.
 

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it seems your model is controlled by analog electronics [and my expertise is limited to mechanical injection]

but the actual cause of the problem would seem to be whatever algorithm/device is used at warmup to do the "choking" control - ie the ratio once warmed up of 14:1 or so needs to be say 6:1 when cold but seems yours is richer than required maybe 3:1 or whatever

so you need to discover what device is used to richen at start - eg on mine it is via the Warm Up Reg ie a bimetal strip etc etc but I imagine yours would simply be a temp sensor feeding into some electronic gizmo BUT the sensor or the gismo has got the algorithm wrong

It may be worth getting the Bosch book for your system then you can explain it to all of us
 

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Tech,

Thank you for your quick reply. I am just using the Haynes manual, which terms that valve underneath and toward the forward end of the intake manifold the air control valve. According to the manual, it bypasses the throttle plate and regulates the idle speed based on input from the ECU. I pulled it off, checked it for freedom of movement, and checked its resistance. It looked good.

The Doctor,

Thank you, also, for your reply. I found out at the parts store that there are two temperature sensors: an engine temperature sensor located under the #2 intake with a one-pin electrical connector and a coolant temperature under the #3 intake with a two-pin connector. Which one do you think might be related to this problem?

I should also add that, because of my tinkering, even though I have improved the air-tightness of hoses and cleaned dirty hoses and valves, my car runs worse now. It always surges at least somewhat, most violently just after starting, but a weaker, nagging oscillation of RPM and lack of power is constantly present.
 

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well as I say you need to understand the system or tinkering is just tinkering but at least it is YOU tinkering and not paying a garage for the normal "let's try this [$200] well that was not it so lets try that [$200] etc etc"

it is far better to buy the Bosch book and read the section for your system and as I found for my system [after paying $1000 and being without car for 2 weeks] it will all become crystal clear because book will tell you what to check in a logical sequence

I actually borrowed the book and copied about 10 pages but from memory it went on to describe the systems after my K Jetronic as well

good luck
 

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Discussion Starter · #6 ·
Where might acquire one of these Bosch manuals?

I figured out that the sensor under the #2 intake with a single electical connection is used by the temperature guage on the instrument panel. It is the sensor under the #3 intake with two electrical connections that is used by the injection system. I its resistance between the terminals, which was 3060Ω cold (57 F) and 302Ω warm, which looks good. It does surge less and run faster with this unplugged, but I am sure that this is just because the the electronics are being told that the engine is cold (resistance is high).

Is there any way to be sure that the mass flow meter is not the culprit? Its resistance between certain terminals was supposed to be between 3.5Ω and 4.0Ω, but mine read 3.1Ω . I did order a refurbished one, but when I tested it, it also had a resistance of 3.1Ω , so I returned it without installing it. I could not afford to commit to a $185 part that may not have been the problem.
 
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