My car has ~190000 miles. After noticing that it felt a little sluggish (I drive almost exclusively on the interstate though) I also started to think I felt the occasional little stumble while at stoplights. Yesterday I started the car and it was misfiring very consistently. Feels like one cylinder is just not working at all. Check Engine Light was blinking, scanned only one code, P0304 "Cylinder #4 Misfire" which seems to agree.

Today I bought a new Volvo replacement coil (figuring at worst I would have a spare) and a new set of genuine Volvo plugs. No change.

So first and foremost; What conditions may exist for a 1996 Volvo 960 wagon to set P0304?
I am going to try and troubleshoot as follows:

• Mechanics stethoscope (or a screwdriver...) to hear for operation of the fuel injector solenoid. Disconnecting the plug seemed to change the way the engine ran, but actually in the same way as all of the other injectors when I unplugged one each.
• If injector doesn't seem to work, check for 12V switched supply and 12V pulse (with ignition fuse pulled - assuming there is an ignition fuse; right?).
• Is it considered awful in the Volvo world to pull the coil with a plug attached and check for an arc to the block? Otherwise how do I check for function of the plug? I suppose I am used to using a timing light on the plug wire or arcing to the block... Old Chevy's...
• The plug did appear dry in #4, and no sign of anything burning after running the stumbling engine for a minute or so. Not too telling though. Piston is filthy looking. Spark plug and plug bore smelled dry of fuel, hard to tell since I did use a little dab of anti-sieze on the plug threads.

I think I may run a compression test as detailed here since I will have each plug out and the engine does have so many miles it would be worth knowing if it won't risk damaging the engine.

PLEASE advise on anything else I should check or any special methods I should use specific to this engine. Thanks much!

 

Try this 1st.
https://volvoforums.com/forum/showpo...29&postcount=2

 

checking compression is never a bad idea if you have the means, though - might find a problem before it acts up, and nip it in the bud.

And I can't speak to old chevy's, but I've done quite a bit on a 68 GMC Sierra with a 350 big block ;-) was so sad to see her go :'-(

 

I did notice that the connector quality seems awful - the insulator inside the connector has turned into a waxy mess and the connector pins do move freely. I did try to jam them into place but it's hard to verify a connection. Might be time for a few new connectors; sure would be great if it's that simple!

All I have read since my first post says the coil on plug systems are too sensitive to just arc to the block, and that a tester with a fixed (but still larger than a sparkplug) gap should be used to stress the ignition system a little. NAPA sells these in at least two models, inline may be preferable since I can test the spark with the plug in the engine. I'd like to be able to verify the entire circuit branch. I figure this and a mechanics stethoscope on the injector will tell me if the computer is driving things correctly.

These images from www.NAPAOnline.com
OR

 

You can disconnect a coil while the engine is idling, it won't harm anything and you will be able to hear the spark jumping, you could also put a spare spark plug in the removed coil and ground it to the head and look for spark color.

As RSPI states, check the connector and wiring first, a very common trouble point.

Next, test the coil, swap the coil with another and see if your miss moves with it.

If so, then the coil is suspect. If not and it still is at the same cylinder, then, look into the power stages, which are located fore and aft of the bulbous portion of the intake manifold. One controls 1, 3 and 5 and the other 2, 4 and 6, I could be wrong on the actual cylinders controlled, though, try just unplugging and replugging and see if corrosion of the connection is the cause. If it is still missing, then swap the power stages and see if the miss moves to another cylinder.

If none of that works, then you could have a bad injector, but I would lean more to a air leak, whether it be a crack in the the short rubber hose connecting two intake parts or around the injector or even an intake gasket leak. One could use propane gas to search out an intake leak, though it requires tremendous caution and to be outdoors.

If you manage to do a compression check that would rule out a bad valve if the readings are normal.

Does this miss occur right after a cold start up? Does it get worse when the engine is hot? Is the engine harder to start when hot/cold?

Injectors can be tested on the car with an ohm meter, should read approx. 16 ohms resistance. That would test for the integrity of the coil in the injector, it would not measure any build up of fuel deposits. Some injector additive might be worth while. By the way, I still have the original injectors on my engine, so they have over 356,000 miles, so the injector are really robust.

DanR '94 964 356,000 miles (122,000 on the new engine)

 

Thanks for the detailed post, Dan550i! I'll reply inline with the text, I'll abbreviate your text some...

I replaced the coil, replaced the connector, and I bought the spark tester shown above (the spark-plug looking one). Nice hot blue spark at cylinder #4. Have not swapped the coils since I REALLY do not want to disturb the other connectors unless absolutely necessary.

If I have a good spark; does this mean I can assume the power stage is functioning properly or should I test this anyways?

Mechanic's stethoscope sounded like all six injectors were functioning, same sound at all six. Additionally, probing about the engine head didn't yield anything too fascinating. I am not sure if I have a WEAK fire or NO fire in the cylinder - I'm not that well tuned with this tool. But those injectors are definitely noisy. I do need to check the internal resistance like you said, it would be another good easy test. A clog in the injector is something I had wondered about, I suppose if it were clogged good it would cause a misfire. One thing I did notice that the engine DID change speed when I unplugged each of the injectors, but the sparkplug can out dry and it didn't smell of fuel in the head. I may have missed the smell and I need to do this again.

I'll check the condition of the vacuum hoses. But can this make a single cylinder misfire code trigger? The thing is - I still don't know what conditions must exist for a P0304 code to trigger. How much does the computer see and what logic does it use to set the code?

Yeah, I tried listening to the valvetrain with the stethoscope but it's too noisy to discern anything. I might have to do this compression test soon since I know I do need new spark plugs.

Miss at all times. It does not seem to be time or temperature dependent (which is the reason I started with spark). The intensity and speed definitely varies with load and engine speed, but that should not be surprising if the other five cylinders are working correctly or nearly correctly. I had to drive the car like from work, it seemed almost perfectly persistent the whole way. It's just rougher at lower engine speeds but I assumed that's harmonics. At stop lights the car rhythmically shakes very noticeably. The car accelerates less quickly than before but it isn't something I felt unsafe with merging into traffic.

Could low fuel line pressure cause this? Maybe I should swap out the filter and also add the recommended injector cleaner. Anyone have any favorite brands or types of cleaner additives for this purpose?

 

This is what I use in *all* my cars on a regular (every couple/few months for older cars) basis - http://www.chevron.com/products/prod...tives/tcp.aspx

When getting a new car I usually run the first 3 or 4 tanks with that stuff in it just in case the previous owners were neglectful. Always helps my mpg and I've even had it help pass a PA emmissions test! (failed, pumped that through my lines, then went back 2 weeks later and it passed )

Lucas might make something good (better/pricier) but this is what I use fwiw

 

I would think you can assume that the power stage is functioning correctly. You have spark and that is too bad, as that may be the simplest fix.

An air leak could cause a localize cylinder code, how that is determined by the computer, I have no idea. But air leaks generally fade off with throttle. I would go to one of the auto parts stores that loans tools and get a compression gauge, if you can find one that does not need to be screwed into the spark plug hole but is just held against the opening, it will be much faster.
It is preferred to have the engine hot, the throttle valve held wide open and the fuel pump fuse removed. If you do that and it turns out not to be a valve problem, then your numbers will be good as a bench mark for possible later use.

Another thing to try, with the ignition off, unplug the MAF, the computer will use substitute values to allow the engine to run. See if it runs better. Also, disconnect that air tube that connects to the air filter box. The damper inside the filter box can get stuck and draw air from the exhaust manifold and fry out a good MAF. Only unplug/plug the MAF with the ignition off.

Does the idle change when you remove the number four coil or disconnect the injector? It should, but if there is no compression then the idle will not change much. When you do unplug injectors, the fall of of power should be consistent for all cylinders.

You have not been able to localize the missing cylinder, while the computer says it is number 4, you should be able to confirm that by removing the coil, or injector plug and not have the engine change tone at #4.
I wonder if you have a bad spark plug? Could it be that simple?

Low fuel pressure would be more of problem at hi rpms or wide open throttle.

DanR

 

I am running on the assumption that cylinder #1 is closest to the timing belt - I suppose now is a good time to make sure that cylinder #4 is the 4th cylinder away from the timing belt... Occurred to me earlier that there's a chance I'm being a dummy.

But anyways, it looks like it's time for the compression test (about 180-190psi on a 10.7:1 engine, right?) Looks like another run for some cheap tools that will work perfectly for a month $20 compression tester kit should do the trick. I'll have results from the compression test (dry and wet values) tomorrow.

Edit: I should answer your questions...

Yes, it does. Seemed to cause a very similar change in engine tone and RPM to unplugging any of the other injectors. I was not sure what to make of that. My answer here is probably not that helpful; the results are fuzzy to me and I need to do this same test tomorrow again before the compression test (may as well do this while I warm up the engine a little).

I installed a new Volvo plug. No change at all. I'll check this all when the plugs are coming out.

 

Yes, number one is at the front.

Let us know the results.

We have auto parts stores that will loan the tools for repairs and diagnosis, just leave a deposit which covers the cost of the tool and get it refunded when it is returned. Have done it man times.

DanR

 

I checked all six cylinders. I decided to wait for the good one until last. Cylinder #4 -- 0 psi. I didn't get a good seal on the others with the conical adapter, but they definitely behaved similarly. One totally shot cylinder system.

There was nothing bad coming out of the dipstick tube with the engine running, nothing unexpected with the oil fill cap removed, and cranking with the sparkplugs out was smooth and quiet, so I am thinking the bottom end is still fine. I might have a stuck or burned valve causing this. Now I am considering a remanufactured head and a nice 12 hours of wrenching. The rest of the car really does behave nice on those looong drives I have to make. 21mpg average is not the best - I got the same with my 1993 Cadillac Fleetwood landyacht.

Looks like I have some thinking to do on this one. I definitely appreciate the help and if I decide to pull the head and replace the headgasket and timing set while I'm in there, I'll likely be asking for more advice. Guess I need to get a complete factory service manual and see what European car horrors await...

 

That was my worst fear, it will very likely be a valve, if you are still driving it, disconnect the injector to that cylinder, it will keep unburned fuel going to the catalytic convertor.

You may not need a head, I would get the head removed, or remove it your self and then upon inspection decide. Most likely just a valve has been broken and in a way that it will be missing an edge that prevents a tight seal. If that is the case a machine shop can replace that one bad valve (cheapest). Have whole head done, valves lapped seats reamed, new valve seals. Or get a re-manufactured head, close to the same cost as getting the whole head done locally.

Interested in what you decide.

DanR

 

I'm parking it until it is fixed properly (or maybe it will just be sold ).

I'll post what I decide. Need to take a good several days to do other things first. Unfortunately I'm not going to be able to move quickly on this due to complete lack of time and another project vehicle much higher on the list.

 

Going to be removing the head and seeing what is going on in there. Timing belt, water pump, flame trap, etc all done at same time. Going to take lots of photos and hopefully have the discipline to document this well

Volvo green book seems to say that the upper head/camshaft cover can be removed while leaving the camshafts in place on the lower head. However, this guide for a 850 (not sure how different that engine is) says the camshafts should some up with the upper head. I just want to be sure there is a good reason to go one way or the other. Right now I lean towards keeping the cams secured to the camshaft cover; when the parts are brought back together it's easy to ensure proper engine and cam alignment this way. Sure seems like more work upfront but a time saver when the fun of working on the engine is wearing thin...

I need to explore that more as it's not covered in the green book. Are there any tricks that may help me keep things aligned and in good order?

 

The cams will have to come out, once the head top is loose, the cams pop up under the tension of several valve springs. Most times though, the glue/sealant requires that the head top be pried off.

There are a few tricks, one is to find and locate the timing marks on the two cams and the crankshaft. The most important part then is place the engine with those marks in alignment, then mark the cams as exhaust and intake and then mark the timing gears to the cams. The timing gear will need to be removed.

Four c-clamps will help with the removal and installation of the head top.

DanR '94 964 356,000 miles (122,000 on the new engine)