Since finally getting my 95 Ram (318)to run well about a month ago, I have since developed a missfire on cylinder #8. I have fiddled with it now for a week and decided to run it past you guys.

It is most noticable when the ram is cold. It has spark. It had an ok compression test about a month and a half ago (# 7 was low, but #8 was ok), and sometimes the miss will go away after it warms up and after I hold down the throttle and keep it at a steady RPM. The miss is mostly on startup and when under load.

I have rechecked the injectors and even swapped the number 2 and 8 injector, but the miss remained in #8.Unplugging the #8 injector while its running didnt seem to make a huge difference except for 1 time when it was running a little better and I heard a little stumble.I also retorqued the intake bolts. The cap and rotor are new, although again I have spark. I might hear a hissing above the driver side exhaust manifold, and it is missing a bolt...but its the wrong side. Cat was gutted, but is possible in theory that a peice a brick was blown into the muffler.

The IAC is possibly suspect as I allowed the pintle to come seperated a month ago, and I shoved it back. But as I said, the truck ran well for a while after that.

Ideas?

 

There can be arcing problems between the #4 and #8 wires especially if the wires are cheap and they are running parallel.

Check the TSB for wire routing and check you routing to see if it doesn't go away.

http://dodgeram.info/tsb/1998/18-48-98/18-48-98-v8.htm

 

The wires are the cheapest, but I did my best to rerout them acording to that TSB today. The #8 wire is isolated now from all the others, except at the spark plug seat where it is close to #6 (not much you can do about that).

Did I mention I had a problem with rust in the fuel rail? The fuel rail is all clean now as are the injectors, but if I had low pressure could the #8 cylender (the farthest) be running too lean?

 

As you can see from your fuel rail clean-out work
cylinders 7 and 8 get fuel first
and so they also get rust particles clogging them first
as well as rubber particles from fuel lines breaking down.

This junk in the fuel injectors of cylinders 7 and 8
can cause them to run a lean air to fuel ratio at full throttle
and this can cause cracks to form in the metal ridge
that runs between the intake and exhaust problem.
Unfortunately these cracks are common in Magnum Cylinder heads
and grow over time from shallow to deep enough to let coolant
start leaking into the cylinder. Coolant leaking in will cause missfires
and leave a white/yellow color deposit on the sparkplug nose.

Another problem is that the intake manifold has a plate and gasket
hidden underneath it (called Plenum gasket) that also commonly
develops a leak near cylinders 7 and 8. Excessive oil being sucked in
near the intake runner mouths for cylinders 7 and 8 can cause missfires and
leave a black/brown colored deposit on the sparkplug nose.

Do your sparkplugs in cylinders 7 or 8 can colored deposits?

You can further check for cracked cylinder heads by doing a 'leakdown test'
using a tool like this:



http://www.area51groupllc.com/produc...products_id/30

 

Hank, a leakdown test was done about a thousand miles back to see why the compression in #7 was only 100. It was inconclusive per the boys at the shop. The engine itself, I would not think would be old enough to develop a crack, it only has 5k miles on it. I really need to get on Jasper for selling me an engine that has a low reading on #7, but that one is firing and I just want to get it running good for now until I guess I make them send a new engine.The plugs look normal.

There is a vacuum line coming out of the EGR control thing (it has a 90 degree angle) that is broken off.Its not one of the ones going to the EGR unit itself. Cant see where the other end is to connect it back up.

 

Of the two lines that come off the EGR valve
one is the 'exhaust backpressure sensing' line.
This one is shorter, fatter, and made of stiffer heat resistant material.
It goes to the "EGR Modulating Valve" that rides unmounted nearby.
and has an electrical plug to it.

The second line is the 'chopped vacuum signal' tube.
It is longer, thinner diameter and slicker surface.
This also goes to the EGR Modulating Valve.

A full strength vacuum line runs from the intake manifold over to the EGR Modulating Valve. The EGR Modulating valve then chops this vacuum strength down to pulses of lower vacuum that can vary, and so only part way lift the EGR Valve pintle so that variable amounts of exhaust gas flow into the intake manifold at the back. The PCM computer does this varying, and judges how well everything is working by reading how the O2 sensor 'jiggles' as EGR changes. It is complicated.