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Heavy misfire on cylinders 3 and 4

C

Chad-brunswick

New member
Joined
Dec 25, 2020
Location
Wyoming
Let me preface this by saying yes, I am pretty slow, but a couple months ago my 1989 740 gl developed a heavy misfire on cylinders 3 & 4. That morning I “accidentally” revved it to just under red line and it seemed fine until I was driving to work and it started stumbling and eventually died. I pushed it into a parking lot, finished my shift and then got it towed home. It then proceeded to sit for a couple months until I could get my uncle to help me look at it. It has good compression, good (I think) spark, and all the injectors are working with decent fuel pressure . We switched out injectors, engine temp sensor, fuel pressure regulator, intake gasket, and throttle body gasket and the problem still persists. I’m at a loss as I’m not much of a mechanic (yet) and my uncle typically works on diesels. Any advice would help thank you.
 
Look closer at the ignition system. Consider running the engine at night with no lights and look for arcing from the plug wires...or if the ignition system has a decent amount of mileage/time since the last tune up go ahead and do that.

This is mostly from my experience of when diesel mechanics work on gas motors they can easily get tripped up on ignition systems.
 
ZVOLV said:
What's the compression?
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If I remember correctly I believe he said it was around 140 ish (or somewhere in that ballpark) on all cylinders, I?ll see about checking the ignition stuff this Saturday though, so long as the weather holds out. This is my first Volvo and second car in general that I?ve had to work on so I appreciate the advice
 
If all of a sudden 2 adjacent holes develop a misfire it is often caused by a blown HG between those 2 holes. No coolant nor oil involved.
You should be able to see lower compression numbers on both holes because 1 cyl robs compression from the other cyl and vice versa.
 
I am curious. Since the car is pre OBD II they generally do not have ECU misfire detection schemes. How did you diagnose that the misfire is on cylinders 3 and 4 as opposed to being just a general misfire? The fact that the engine died and you had to tow the vehicle home, does that mean that you have now progressed from a misfire to two permanently dead cylinders and the engine does not run or you can get it to run; but, badly?

If you have two permanently dead cylinders, I would first confirm that you have spark on those cylinders since you seem to be uncertain about that.

If you have spark, then you need to review the compression test results to confirm the numbers. If you have access to a compressed air supply you can try pressurizing the affected cylinders at TDC and then listen for air leaks which would be an indication of a HG problem, damaged valves or rings. As an observation, bad compression does not usually result in misfire symptoms, just an engine that has poor performance and can be rough if the cylinders are really bad.

You said the fuel pressure is decent. What is the actual fuel pressure and is it per the spec in the Volvo service manual? That said, low fuel pressure should cause problems on all cylinders, not just two cylinders.
 
Boughricord (sp) Plug wires?

Get some good Taylor or Belden wires....

But before spending money test your wires.

If you don't have an ignition problem, do a leak down test looking for a gasket leak between the cylinders?
Or Intake leak near those runners?
Does it miss only at idle or under power as well?

You did not indicate spark plug visual inspection results? Really this is the first place to look for clues to what may be borked.
 
Dirty Rick said:
Boughricord (sp) Plug wires?

Get some good Taylor or Belden wires....

But before spending money test your wires.

If you don't have an ignition problem, do a leak down test looking for a gasket leak between the cylinders?
Or Intake leak near those runners?
Does it miss only at idle or under power as well?

You did not indicate spark plug visual inspection results? Really this is the first place to look for clues to what may be borked.
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It has brand new plugs and wires about a couple hundred miles ago, new intake manifold gasket, and if I remember correctly about 14.6 or so psi of fuel pressure, it misses on all conditions and doesn?t rev past about 15 grand. We looked at the exhaust manifold through a thermal camera to determine that the back two cylinders weren?t firing. I?m beginning to think it?s a distributor or the coil or the spark module, but we took all the pugs out and set them on the manifold and they all fired. This is begging to seem like a mystery to me. I think once I get some money saved up I?ll probably pull the head and do a visual inspection of the head gasket and replace it and most of the big ignition stuff(cap, rotor, coil, module) I can only really get to it every couple saturdays as it?s not at home, but I?m really hoping to get it figured out. It?s been my favorite car to drive full stop, and to be fair I did abuse it quite a bit, but I think from now on I?m going to baby it. That is until I can get ahold of a newer Volvo or some a station wagon of some sorts, then I?ll build it to the teeth, but first things first I need to get it back on the road. And if I can?t figure it out I have a 79 f150 that I might start throwing money towards, but I?d seriously prefer to get my Volvo running because in my opinion the 740 is one of the most beautiful cars on the road.
 
Did you do a compression test yet? That will tell you if you need to pull the head or not. Don't just throw parts at it. You'll have a lot of trouble figuring it out if you do.

Are the spark plug wires in the correct order? It's easy to get them wrong on the head mount distributor.
 
MasterBlaster said:
Uhhhhhh..... :wtf:
I hope you flipped some digits, and you meant 41.6?
Measured where?


:run:
15 hundred, perhaps?
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I was using psi because I?m a dumb yank. It was probably around 36 kilos or whatever, measuring from the cold start injector because we had to rent the tool and it didn?t have the correct fittings. you knew what I meant with the rpm. It?s been a long day. And yes the plugs are in the right order and we did do a compression test but I think we only tested the back two cylinders as they were suspect. But like, I?m still new to volvos so I?m unaware of all the specifics and what not. I don?t mean to sound retarded but to be fair I kinda am.
 
There are two very important ground wire connections on the intake manifold. They are held down with the bolts for the fuel rail. One is for he fuel injection system the other is for the ignition. Be sure both of them are clean and tight.

An 89 740gl should be LH2.4 and you should check the codes. They may help solve what is going on.
 
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dl242gt said:
There are two very important ground wire connections on the intake manifold. They are held down with the bolts for the fuel rail. One is for he fuel injection system the other is for the ignition. Be sure both of them are clean and tight.

An 89 740gl should be LH2.4 and you should check the codes. They may help solve what is going on.
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I?ll be sure to clean those grounds next time I?m out, I suspected it might have something to do with those but it hasn?t been throwing and codes, and my apologies for the confusion with the fuel pressure, I just don?t remember the specifics off the top of my head, I just recall my uncle saying that it was in spec, and I doubt the front row cylinders would be running with that low of pressure. I?m not sure though. I might have to rent a tester and re check or consult my uncle and see if he remembers. I will update y?all when I find out more
 
Chad-brunswick said:
I?ll be sure to clean those grounds next time I?m out, I suspected it might have something to do with those but it hasn?t been throwing and codes, and my apologies for the confusion with the fuel pressure, I just don?t remember the specifics off the top of my head, I just recall my uncle saying that it was in spec, and I doubt the front row cylinders would be running with that low of pressure. I?m not sure though. I might have to rent a tester and re check or consult my uncle and see if he remembers. I will update y?all when I find out more
Click to expand...

So I cleaned the grounds and that wasn?t the issue, I think my next course of action is going to be to check the distributor and maybe check the plugs. If that doesn?t work I?ll move on to doing another compression test hopefully, so long as we can get ahold of the tool. Thanks for all you guys help so far. I really want to get this beauty back on the road
 
Get a vacuum gauge and read the vacuum. I like the head gasket theory. I blew the hg between 2-3 and had similar issues. Vacuum went to 0-10 when running.
 
I replaced the crank position sensor and now it?s actually trying to run on all four cylinders. Before I?m pretty sure the back two were dead but I?m getting closer to figuring it out. I actually got some codes now and they?re coming back to the coolant temp sensor that I replaced so I think the next course of action will be to verify the wiring and vacuum tubes and stuff like that
 
Just did the head gasket the other day and absolutely no change. This is really starting to piss me off. I am honestly stumped. The only codes I?m getting are for the ect which I fixed but I?m still getting codes for. I need to get this car figured out but nothing I do does anything. I just want to sell it but I?ve put too much into it to sell it for 500 dollars.
 
Never replace parts without doing circuit verification. You need to make sure the circuit isn't high resistance/open circuit going from the sensor to the computer. One wire goes to the fuel box, and one wire from the temp sensor goes to the ignition box. It grounds via the cylinder head.


The rear sensor is for the computers. The front sensor is for the temperature guage.


I would start with unplugging the rear sensor and then see if the car runs ok.


I may also unplug the MAF and see if the car runs OK.
 
ZVOLV said:
Never replace parts without doing circuit verification. You need to make sure the circuit isn't high resistance/open circuit going from the sensor to the computer. One wire goes to the fuel box, and one wire from the temp sensor goes to the ignition box. It grounds via the cylinder head.


The rear sensor is for the computers. The front sensor is for the temperature guage.


I would start with unplugging the rear sensor and then see if the car runs ok.


I may also unplug the MAF and see if the car runs OK.
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I already unplugged and replaced both those sensors and it ran the same. If not worse. I haven?t tested the wiring yet but the wires are okay after a visual inspection. I ordered a distributor coil and ignition module and if that doesn?t work then I?ll look at the wiring again
 
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