Miss firing

[dvs bullet]

How much vacuum is 5.0 supposed to have in the intake manifold? I've got a real stubborn miss that intermittant, and I've checked and replaced all the ignition componants (plugs, wires, cap, rotor etc.) so I think that I have ruled out that. The next thought could be that, I have a vacuum leak or maybe a fouled injector. Any thought or suggestions? thanks in advance

 

[dvs bullet]

any one?

 

[94_Teal_GT]

My 86 5.0L Merc has a vacuum gauge mounted under the hood. Previous owner used it as a tuning tool and decided to leave it there. But anyway, at idle the Merc has a rock steady 21 in.Hg with 242k miles.

Just checked my gauge, it says
0-11 in.Hg is Late Valve Timing or Vac Leak at Intake or Heat Riser
11-15 in.Hg says nothing
15-18 in.Hg is Late Ignition Timing
18-22 in.Hg is 'Normal'

 

[dvs bullet]

thanks that helps alot

 

[dastang2]

also a bad head gasket could do this too. do a cooling system pressure test.

 

[dvs bullet]

more info, I just a set of plugs on her and they all came out clean. A friend and I checked around the intake with acetaline, seemed like there was no difference. The vacume is at a steady 17, and the only time that the idle picked up is when when we put the acetaline into the diapham of the egr, it ran awesome. We changed the egr thinking that it was leaking, no luck, the miss was still there.

 

[cschulzy]

I had a missfire problem on my stock 89 GT 5 speed. I was running splitfire plugs at the time and changed them for stock motorcraft ones, but the problem was still there. As it turned out the old plugs were well worn and I had gapped the new ones to wide. For a cheap fix it might be worth checking your plug gap. My missfire was under load from 1500 rpm up at anything past 3/4 throttle. Hope this helps

Cheers

Chris

 

[dvs bullet]

I had a missfire problem on my stock 89 GT 5 speed. I was running splitfire plugs at the time and changed them for stock motorcraft ones, but the problem was still there. As it turned out the old plugs were well worn and I had gapped the new ones to wide. For a cheap fix it might be worth checking your plug gap. My missfire was under load from 1500 rpm up at anything past 3/4 throttle. Hope this helps

Cheers

Chris

I'll give that a shot today, thanx

 

[dvs bullet]

just an update, again I found a vacuum leak under the plenum of the car (One of the pcv hoses were cracked), this restored the car to 20 inches of vacuum, we then set the timing on it to 12 deg. I also double checked the plug gaps. The car idles smoother but you can still feel a bump in rpm, I also went along with a stethascope to all the injector and all of them were clicking. The plugs that came out before were a light brown colour so there isn't any evidence of a head gasket ( also the engine was rebuilt 10,000km's ago).Any more Ideas? thanks

 

[dvs bullet]

none?