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The quick answer:
Does the exhaust smell rich when warmed up? Maybe your cold start injector is leaking (a lot)? Also, the control pressure regulator on the L-jetronic can fail at around 140,xxx miles on a twenty year young 240, also, causing fuel to dump into the intake port, making for some seriously rich fuel mixture.
The control pressure regulator sets at the fuel injector rail front secure by two bolts. At the front is a short vacuum line that connects to the intake port. When the control pressure regulator fails, the diaphragm separating the fuel under pressure from the atmosphere (under relative vacuum as part of the engine air intake environment) ruptures, and fuel can run rampant into the intake port. Pull the vacuum line to check for this leak.
The longer systemic answer:
A weak spark from a failing or poorly connected ignition system can cause a rich running condition, probably.
Yes, check for vacuum leaks on the intake side. The usual suspects like the plastic accordion-style intake hose. It'd have to be a pretty huge hole to be running extremely rich or lean per code 2-3-1 that sets the CEL (check engine light).
If your 1989 Volvo 240 (presume all stock under the bonnet) engine is running extremely lean, it can cause some pre-ignition ping well beyond what the ignition control computer module can do by changing the timing when it picks up valve ping through the ignition system knock sensor. So, you'd hear the ping with a warmed up engine when you accelerate.
You should check the EZ-1xxK ignition computer module codes, also. Connect the OBD box lead to socket six and proceed the same as your would when checking the L-Jetronic computer module for any stored fault codes. Research any ignition code faults as there may be a fault corresponding to a rich running condition.
Hope this helps.
Happy Holidayze,
kitty is a fuzzy one driving the grey volvo 240.
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