I’ve had my Volvo for a couple months ago, when I got it, it was sitting for at least 2 years. It was having trouble starting so I replaced the battery, MAF, intank fuel pump, fuel filter, cleaned the injectors(made sure spray pattern was good), replaced the Knock sensor, coolant temp sensor, crankshaft position sensor, ignition module, spark plugs, ignition coil wires(not the ignition coil yet but voltage was checked), cleaned the rotor and every connection possible, the harmonic balancer was shot so it got replaced, timing was checked and it was off a good amount, I had a friend reset timing but it still has the same issue. At night when the cars cold and it’s colder outside the car runs absolutely amazing but then around 12pm when I’m going home from college it bogs down and backfires into the exhaust, and air intake. I’ve tried absolutely everything and I have no clue what to do next. If anyone has any suggestions let me know!


-josh

 

I suspect you have a fuel mix issue. What year is your car and what FI system do you have? have you inspected intake air boots and vacuum hoses for cracks and sprayed around components to see if there's a intake air leak? You may want to have a smoke test done since leaks can be hard to find. Have you tested the O2 sensor and checked wiring/connectors? You may have a weak connection that turns into an open circuit when the metal heats up, Final area is the fuel pressure. Have you measured pressue up front both cold and warm? Could be the pump is getting hot and losing pumping power or the relay etc. you may want to measure the current draw of the pump to see if its in spec.

 

An exhaust back fire is usually caused by a very rich mixture. The unburnt fuel gets into the exhaust and explodes there. Backfiring in the intake is usually caused by a lean mixture. The lean mixture explodes in the intake manifold. You are probably not going to be lean one minute then rich the next. Both backfire types can be cause by the spark plugs firing at the wrong time. (when the intake or exhaust valves are open)

You - " Replaced crankshaft position sensor, timing was checked and it was off a good amount, I had a friend reset timing"

You can't "set the timing" on your engine. The ignition timing is controlled by the crank position sensor and the Ignition control unit. Older cars you would turn the distributor to set the ignition timing because the points were triggered by a cam in the distributor, then we switched to a hall sensor in the distributor. Same thing, twist distributor to set the ignition timing. There is nothing in your distributor other than a rotor button. The distributor HAD a plastic stop to hold it in the correct position when first installed. That must no longer be there. If so it's possible the distributor was turned enough to not have the poles in the distributor cap line up with the rotor properly - and perhaps at times the spark jumps to a different pole in the cap and to the wrong plug - causing the backfires. Or the cap has carbon tracking in it and needs to be replaced.

Set the engine on TDC firing on #1. (line up the crank and cam - the cam mark might actually be firing on #4 - it's been a while since i've looked) Remove distributor cap. The rotor button should be lined up with the mark on the top edge of the aluminum of the distributor.

 

It’s a 1992 Volvo 240 that is fuel injected. I’ve looked for every possible vacuum leak and haven’t found any. I checked the o2 sensor and there’s nothing wrong with it. The fuel pressure is good, but I only checked it when it was warm, the draw from the fuel pump is in spec. I started with fuel but after looking I can see that it definitely is dumping fuel.

 

I meant to say we set the cam and crank timing not the ignition timing. And yes I do not have an older distributor cap it cannot be moved. I’ll have to check for that plastic piece and see if it’s missing or not. I’ll have to get a video of what’s going on, because when it is idleing yes it does just backfire from the exhaust but when it is Under load or press the gs pedal down at all it just backfires from the exhaust and from the engine bay from what I can hear. I’ll have to get a video to explain. This is why I’m confused because I’ve never had this type of issue happen before. I’ll get back to with some answers I’m working on it tonight after work. Thank you

-josh

 

Then the mixture is off due to fuel pressure, bad air mass meter, bad sensor signals, etc. Or the cap is tracking and the spark is going to the wrong posts inside or outside the cap.