As some of you already know, I have been battling with my emissions for about a month now. There are several problems as to why it won't pass but I think I found the worst of them yesterday. I was getting my timing adjusted and the mechanic discovered that my truck is only firing on 6 cylinders. Cylinders 7 and 8(2 closest to the driver) are not firing. We tested this theory by pulling the plug wires and there was NO change to the way the engine ran. The suspected culprit was a blown head gasket, but upon compression check we proved this to be wrong. Cylinders 6 7 and 8 all had compression at 130 psi. I assume the explanation for my HC levels(1600's) is that raw fuel is going into these 2 cylinders and not getting ignited. So now that I have tested compression and know it is not the head gasket, what do I look at? I do have major vacuum issues which I am working on, but would disconnected vacuum lines actually shut off 2 cylinders?? Any help is appreciated.


Keith
~Houston, TX

 

i would start with new sparkplugs along with distributer cap, rotor, and sparkplug wires

 

might also look into a new msd coil while your at it.

 

Whoops, I guess I forgot to mention I just did a tune-up. Oil Change/new filter, Air cleaner, Plugs, Cap, Rotor, Timing adjusted...sorry!


Keith
~Houston, TX

 

are plug wires 7 & 8 crossed? if so these are notorius for cross firing wich could also be creating the problem. what kinda plugs are you using?

 

8mm Wires, Motorcraft plugs. All wires are correct.

 

Sounds like lean misfire, look for vacuum leak. If you don't notice power loss the misfire probably is at idle or low rpm.
Is high HC at idle speed.

 

Crossfire between #7 & #8 spark plug wires cause ping and drivability problems.
searched and found this article..

sounds like the wires can be in the right location (distributer cap to plug)but can still crossfire..

How does one spark plug wire cause another spark plug wire to fire?
Believe it or not the condition you're referring to is a rule of electricity and the principle of that rule is used to make the spark for the ignition at the ignition coil. Whenever electrical current travels through a conductor/wire a magnetic field is formed around that wire. If you place another wire next to the first, nothing really happens until the electricity/current stops moving thorough the first wire. When current stops moving the magnetic field collapses. When invisible lines (called magnetic flux) of the magnetic field collapse they will cut through the second wire. Here's where it gets really cool. As the lines of flux collapse and cut through the second wire, it just happens to create electricity in that wire. This is called mutual induction. Mutual induction is applied in many aspects of or lives, cars, computers, home appliances are just a few items that rely on this rule of electricity. Now, I know anyone that has looked under the hood of a vehicle would say how can this happen? After all nearly all the plug wires run side by side, so there should be more problems. That is true, but for the problem to occur and affect engine operation you need to add a few more pieces to the puzzle. Ford did that by having two cylinder firing events close to each other. To make power with an engine, air and fuel are mixed, compressed and spark is added at a very precise time. It just so happens in some Ford engines the next event getting ready to happen is in the next cylinder. If the spark plug wires are routed side by side the first cylinder receiving spark just happens to induce a voltage that is great enough to cause a spark at the plug in the next cylinder. The problem is, it is at the wrong time! This causes all kinds of disorder. This condition can cause spark knock, engine misfire, false computer codes and driveability problems. If driven under these conditions for a prolonged time it is possible for engine damage to occur. You would know your engine is not running right and would find it very difficult to ignore. Proper plug wire routing is important.
try to keep 7 and 8 wires apart from each other as much as possible and see if that makes a difference...

 

Could be a vacuum leak. I'd also check the EGR valve.

Pat

 

Well, I am not sure what year yours is, but if it is fuel injected I would think it is bad injectors. I had the same problem with cylinder #6. I replaced the injector and now it works fine.

 

I'm with Kam, check your injectors. Easy check, take a long screwdriver and put it up against the injector for cylinders 7 & 8 (back two on driver's side) with the engine running. Place your ear against the end of the screwdriver and listen for the telltale tick tick tick of the fuel injector. If you hear it, then compare it to #5 (forward most on driver's side) to see if you hear a difference in the speed and volume of the ticks. If you hear nothing on #8 your injector is probably dead. If you hear #8 ticking but #5 is much louder, your injector is severely fouled. A true injector cleaning (can attached to fuel rail, not bottle in the tank) may help.

 

A vacuum leak or a bad EGR will not only effect #7/#8 cylinders, it would effect them all. I would look into the fuel injectors. Have you verified that you do have spark on #7,8? Even though you have a new dizzy cap, it could be bad from the get go.

 

I believe Dvlracer has a carbed motor. The problem could be as simple as the plug wires or a bad distributor cap or rotor. You can try switching one plug wire from another cylinder that will reach to check for spark before going out and buying new ones. Just hook it up in place of the non-firing plug wire. Try that and post the results.

 

I have a 351W, w/ a stock 2 barrel carburetor. I checked my inspection report and found that I passed the HC test with flying colors at 2700 rpm. Yet I failed MISERABLY at 1100 rpm. This leads me to believe that when the engine is running faster, all cylinders are firing. (higher rpm=more vacuum, correct?) About the cap and wires, I'm almost positive it's not the cap, b/c it wasn't running any better before I replaced the old cap. As for the wires, I will check this morning. My manual also has a test for the EGR valve so I guess I'll get that done early this morning too. Sorry for not stating all the facts upfront.


Keith
~Houston, TX

 

Your distributor has a "Vacuum Advance" that adjusts the timing as rpm's increase. Either check the vacuum to it and/or replace it. That can cause you all sorts of problems. Keep checking the other items, you may have more than one needing attention. What about the spark test?