Working with a f150 85' with a 6 cyl in think 300 4X4.

This issue: Getting spark to cyl 1 and 2 yet no power. When I pull the plug wires off 1 and 2 I get no change in RPM as I do with 3-6. Also If i start the engine with these wires disconnected I get no change in engine performance.

Why would these 2 cylinders not work even though they're sparking and seem to be getting fuel? Also did a compression test and I got a solid 120 on all cylinders...

Any ideas?

 

are the plugs fouled? are the wires good?

 

Are the valves burnt or stuck? Do a compression test followed by a leak-down test.

 

did a compression test and they were all around 120. plugs are new and wires seem to be good on the 2 that arent working.

Guess I'll do a leak down test once if figure out how exactly that's done...

Thanks again!

 

The "official" way of doing a leak-down test requires multiple gauges and you measure the percentage loss. Might be able to rent such an assembly.

A poor-man's way that merely checks for leaks uses a hose from a compression tester; provided it doesn't have a check valve in it (very important) you can hook it up to an air compressor and use that to fill each cylinder with air. Then, listen in the carb, in the exhaust, in the oil-fill hole, and watch for bubbles in the radiator.

Air from the carb & exhaust is from leaking valves; from the oil fill hole is leaking past the rings; bubbles in the radiator is most likely a blown head gasket.

BTW don't ignore Charlie's advice, also check the obvious.

 

easy way for the head gasket. Gotta have a coolant pressure tester. hook it up, start engine...if pressure maxes out, you have problem.

Also, intake issue...get some spray carb cleaner and spray around 1-2 intake runner/gasket...get a rise in idle, then bingo.

how does engine sound when starting? have a "flat spot" in how it sounds?

 

richsbca,

Welcome to the forum. You are new to the forum, so you don't realize that some of the members who have responded are well known and well respected for their advice. Thank them and check it out.

You said that the sparkplug wires on the two missing cylinders "seem to be good". An easy no cost test would be to swap [one at a time] the wires from this end of the engine to the other end [swap #1 wire for #6 wire] and see if the situation changes.

 

Rich,
Have you ohm'd out the spark plug wires yet? I do mine every 5 years or so. What I do is measure the ohm's and divide by the length, they should all fall within a range (determined by the manufacturer), but pretty close to each other. I buy the Napa Beldin lifetime ones, and exchange them once they go out of spec. (which is usually every couple of years).
If you are getting no change in rpm when you remove one, chances are that is is bad (that is if the park plugs are good and have the proper gap).

 

The wires have been swapped for others with no change. The wires have continuity (6-12 ohm depending on length). I also pulled a the plug wires and inserted a new plug to observe the spark from the plug (and sometimes to my hand!) so I know the plug wire is good as well as the plugs.

Just wondering if it's possible to get a good compression test with a bad valve or rings, etc? Anyway, I think I get the idea for the leak test and will rig something up using less then 100psi so I don't make it worse.

I did notice that when I spray MAF sensor cleaner around the base of the 1 barrell carb there is a small RPM increase. Guess a good vac leak could cause this as well..

thanks again

 

That is true, and I would check all of the vacuum lines and caps and replace them if I even suspected that they might look bad. I do mine regularly, and even keep a box of assorted vacuum caps in my glove-box.

 

use a pressure regulator to add air to the cyl. Never add more than 60 lbs, you can do harm. I agree with vacuum leak, spray carb cleaner all around the intake to head seal, a warped intake can leak, and the exhaust not leak at the same time. It seams odd that 2 cyl together are not firing, you should have a horable miss, to the point that the whole truck rocks.


Another thought because of not really missing, would be to check for carbon tracking inside the cap, maybe it is firing on the cyl, but crossing over to another, so you are not getting a whole amount of voltage to #1, and #2

 

My friend's truck had the same problem, and it was a bad gasket under the carb causing the front cylinders not to get their fuel and run lean. We found it quite by accident when the carb fell off due to the bolts loosing up, turn left and the carb would fall over, sharp turn to the right usually set the cab back down and it ran on! New gasket and bolt/nuts and full tune up cured it.


And yes some engines will run with out shaking when 2 cyliders are not fireing, it depends on engine fireing order,crank torque balance and which two cylinders are not firing.

 

I just went out and pulled #1, and #2 wires off my 89 I6, to see if you are right about the truck not missing, or shaking. It shook and missed so bad, I thought that it was going to rip out the motor mounts.

 

I think I may have found the issue...

missing bolt on the intake of cyl 1. Also replace carb to spacer gasket and this resolved the leak in this area.