The meat:
Cylinder 8 is not operating properly. I have spark and gas, but I don't think it's burning. What else can I look at?

The history:
The car starts easily, but the exhaust has a low frequency beat to it, similar to a Harley. I connected a Tech II and looked through all the engine data and didn't find anything unusual, but then again, I didn't exactly know what to look for. I'm getting no codes.

As the engine heats up, it seems a little better. I took it for a drive and I thought I could feel a rapid pulsation (bump) in the car. Then, the engine felt rough, like it was hiccuping, then back to the "normal" subtle pulsation. I have not driven it since, but here's what I have done...

I pulled all the spark plugs and #8 looked very dark compared to the rest. I replaced all the spark plugs and wires with new ones. Since I have extra plugs now, I unhooked #8 wire from the plug and connected it to spare plug and let it find ground on the block. I started the car and it had the exact same Harley beat, so there was no change with the #8 cylinder disabled and the loose plug had a strong blue spark.

The exhaust has a very strong gas smell.

What could cause the cylinder to miss if I have gas and spark? The only things I can think of sound expensive and beyond the reach of an amateur do-it-yourself. Could I be so lucky that its only the injector dumping way too much fuel in?

Thanks in advance for any tips or advice,
-Michael

 

All thats required for combustion is air, fuel and spark, so if you know you have spark and the intake valve is opening then you have fuel and air so you must have combustion.

There is a table that will allow you to monitor misfire info, it will look like a bar graph, monitor this while the engine is running and if a cylinder misfires then that section of the bar graph will show a block

I won't comment on the pulsation or sound your car is making as I'm not there to experience it but it's your car and you know if it's not running or sounding normal, so I'll just take your word for it that something is not right.

You don't have to ground the plug to the block to get it to fire, you should just be able to plug it in and crank the engine to watch it fire but be warned unless you also unplug the injector to that cylinder, if the engine starts while your checking for spark then you'll also be dumping fuel into the cylinder and pumping raw fuel and air both into the exhaust and out the spark plug hole.

Take it to a qualified mechanic and if they tell you something that sounds expensive then take it somewhere else to get a second opinion.

 

Pull the valve cover, very easy to do. You seem to be describing a broken valve spring.

 

Check your rocker arm bolts. Did you take them off or change your rocker arms? Make sure you used lock tight or else you will get oil in the intake that will drip on to the plug and foul it out

 

Or a bent pushrod or valve. Pushrod and/or spring = easy fix. Bent valve = lots of work.

 

You need to do a compression test on that cylinder! Do the one next to it too and compare the two pressures. If 8 is low, yopu have a bent push rod, bent valve or bad head gasket.

BC

 

Whoo-hoo! Thanks for all the replies and great advice. I dug under the valve cover and found a broken spring. It's on the 8th cylinder as expected.

You guys rule!

-Michael

 

Glad you found it... what do I win for pointing you in the right direction.

 

I hope the valve didn't drop. That would make a mess. Did it stay up? After you replace the spring I would do what Bill suggested and do a leakdown and compression test to make sure the valve is not bent or worse. I hope this turns out ok.

 

The valve is still up. I picked up a spring compression tool today and an air fitting to compress the cylinder to hold the valve up during the change, but I'm still looking for a spring.

I had REV1116's (now called VS-8890's). I called them today to try to find a local dealer and they're going to call me back.

If this change turns out easy for me. I should probably change them all.

Hey 90 droptop, I owe you. Next time you're in the Bay Area, give me a call and I'll buy you lunch. You can reach me through TopDownTech.com.

Thanks,
-Michael

 

I'll take you up on that. I get up there every couple of months. FYI, get rid of the REV springs. Pop the $300 and buy a good spring like Patriot Golds. Consider the fact that you did not drop the valve this time, there will be a next time with the REV's, and you may not be so lucky next time. Just my .02 from experiance.