The 300 six in my truck has a nasty misfire in cylinders 4 and 5. It was running fine when I got home from work last night, but when I started it up this morning it struggled to crank and when it started idled and ran really rough. A few days ago I know I accidentally put 5 gallons of old gas that I'd drained from a parts car in the tank (picked up the wrong can). I realized the mistake and filled the tank up right away to dilute the old garbage. It ran fine for two days, so I doubt this has anything to do with it, but it was worth mentioning. Just in case I also swapped the fuel filter this afternoon, which did nothing. The truck was tuned up with new plugs, wires, cap, rotor, PCV valve, etc. and also got a new offy intake, hedman header, and Holley 390 carb about six weeks ago. It had been running perfectly. I pulled the plugs today. All look good except for 4 and 5, which were dark and wet. No deposits or anything. I checked the cap and rotor to make sure there was no damage inside or cracks or anything. They're fine. I also took wires off another truck and ran them on the 4 and 5 cylinders to make sure the new wires hadn't shorted or something. I know two cylinders misfiring side by side can mean bad head gasket. I'm not getting oil and coolant mixing though. Any other way to tell? I figure a compression check is next, but I don't have the tools...have to borrow a gauge. Any ideas or help is appreciated. Thanks.
Yep, Check compression. Did you change plugs? I have had plugs that look good go bad for no apparent reason. The one weak spot on the 300's was the head gasket, if the motor gets lugged a bunch (I know, no one ever does that on a big 6 ) they will eat the head gaskets. Hopefully that isn't your problem, but it isn't terribly hard to change on a 300.
Compression check will be a good idea . I would also re check the distributor cap for cracks . Make sure your plug wires didn't come lose. You might want to replace those two wires with the old wires to check them and even the cap to eliminate those two possibilities . I know they are new parts but new parts from china , mexico and who knows where can go up at any time . I don't think a little **** gas would do that . Also make sure your valves or lifters are sticking or just plain stuck . That will carbon the plugs too . Those 300 engines weren't good on lifters . Retro Jim
Thanks guys. I got my hands on a compression gauge, but don't have the right size thread fitting for my motor. Hoping Napa has one in stock this morning. I already tried swapping two other wires on and reinspected the cap with no success. I'll pick up a couple of fresh plugs this morning too and see if that helps. This thing does get lugged quite a bit - it's my tow vehicle, parts chaser, and dailly beater. It's a workhorse!
This may be relevant or a red herring - but relatively easy to check. I put some old gas in my Y-block F100. it ran ok but had a misfire the next day. On investigation I found it had bent a pushrod. Changed the pushrod and the new one bent again on the next cold start. Found one valve had gone tight and once I had freed it off all was well again. It may be worth while pulling the rocker cover and checking that all the valves are moving freely and the pushrods are ok. This might not be what has happened to you but it's quicker and easier to check than pulling the head which may be the next logical step. Mart.
Checked the firing order last night beebeebobby...it was correct. I'll take a look at those valves tonight Mart. Hopefully by this evening I'll also have the correct adaptor for the compression tester and get some readings on the cylinders too. Thanks again guys.
Well Mart, you were right. Two bent pushrods. So, how did you free this all up?! Thanks again for all the help guys.
As long as you're in there, bolt on a set of Chev inline 6 rockers. It's the equivalent of adding a RV cam, but for only $35. Who doesn't like more torque?
Result! Well, mine was a 292 V8 but the principle is the same. The pushrods bent because the valves have gone tight - probably as a result of the ****ty gas. Don't know why, but it probably is. Here's what I did. I fed a length of rope down the plug hole and brought the piston up the bore until it stopped by compressing the rope at the top of the bore / chamber. I compressed the spring and removed the spring and collets. I then lowered the piston just an inch or so and lubed the valve (just wd40) and then gently tapped it downwards. Then I brought the piston back up to push the valve back up. Once I had done this a few times the valve became freer and I was able to rotate the valve with a battery drill while applying loads of wd 40 or oil - can't quite remember now. When it was quite free I held the valve up with the piston and refitted the spring. Repeated for the other valve, gave all the others a little press down to make sure they were all ok, liberally lubed the valves and ****oned it up. I added a bottle of fuel additive (redex or similar) to the fuel and it was fine after. Hope this works for you. - Good Luck. Remembering back now I used a big ring spanner and a piece of tubing as a spring compressor, and pivoted it on a piece of chain wrapped around the exhaust manifold. A very Heath Robinson affair but it did work. Mart.
Hey, thanks for all the help guys. She's running smooth as silk now, better than it ever has before. Thank you sirs!
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