Weird engine problem, but cured.

[TallPaul]

Recently had new plugs installed in the motorhome. Took a 200 mile trip and when I got back I noticed sitting at a light that the motorhome was shaking some, kind of like a rough idle, but the tachometer was rock steady. Put it in neutral and the shake was gone. Also notice some roughness when accelerating from a stop slowly, but if I give it a lot of pedal it seems to smooth out right away. Also smooth while driving along the road. With the hood open running in gear the engine was shaking.

So the other day I found one spark plug wire that was wrapped down across the exhaust manifold (actually nearly pinched between the exhaust manifold and the engine cover). I moved it back above and then drove 20 miles city traffic and the engine was smooth at idle in gear. Strange how a little thing like that could affect it so much--if that was the problem, or maybe it was something else that just happened to go away at the same time.

Anybody care to explain how the hot manifold or the pinch was affecting it only at idle and only in gear. It was the driver's side bank that had the wire over the manifold. Maybe in gear the torque caused the engine to pinch it more, but why not at speed?

 

[Gary Allan]

The massive power of your big V8 gives it the impression of being smooth ..even if 6 out of the 8 cylinders are only @ 80% power output.

Using an insulated set of plyers, pull the wires one at a time out of the cap and watch the rpm drop (replacing the one you pulled before yanking another). You will probably find that there are two or three cylinders at idle that are carrying most of the load. Big engines can mask these idle issues better than a 1.6 liter engine that barely has enough power to keep running. I'd venture that the one with the wire pinched is one of the good/better ones.

 

[Jim 5]

Assuming you didn't damage that wire, it was sitting on a probably the hottest place in your engine compartment..the exhaust manifold.

The only thing I can think of is the fact that heat dramatically increases the resistance of most metals. Spark plug wires have a set resistance per foot of wire. If you doubled that resistance for instance, you could create a weak spark at the plug.

That makes some sense to me.....but I'm not certain.

 

[kenw]

YOur wire was arcing to the metal (you just couldn't see it in daylight).

When your engine was under partial or light load like neutral, the weak spark in that one cylinder didn't really hurt anything. However under load it needs all it can so the reduced spark caused a misfire (most was bled off at the crimped place).

This is a common synmptom of a bad plug wire. In your case it wasn't really "bad" but the crimp caused the same results. It might be wise to replace that wire as the insulation on it has been burned and there are now many microscopic holes in it. Humid/damp weather may make it "leak" spark again.

 

[brianl703]

If this motorhome has an SEFI Ford engine with EEC-IV there's a cylinder balance test built right into the engine computer that you can run on it. All you need is a paperclip and a test light and these instructions.

 

[kenw]

expanding a bit....

when you crimped that wire, the insulation thickness was reduced. Insulation is measured in "kilovolts per inch", so compressing the insulation reduced its thickness, therefore allowing it to arc to ground. As I mentioned earlier, the arcing created millions of tiny holes in the insulation, so just "uncrimping" it won't fix the problem permanently.

It's wire replacement time. The microscopic holes are permanent and they will leak again. Perhaps it can be repaired with silicone rtv, and some hi-temp heatshrink, I've done it on 50kv wires with good results, but they were NOT subject to underhood conditions.

Now as to the tach reading solid, the voltage to the plug was not interrupted just weakened so the tach still saw a voltage to trigger upon. You had spark, just not enough under load.

good luck and let us know what happens.

 

[ikeepmychevytoo]

Arching couldn't be noticable even on a v6. I discovered a bad arching only from a ticking sound underhood, it was visible at daylight. Power loss was too insignificant, idle was smooth. Go figure a v8. Yours probably practically grounded itself before the plug.

Speed may cool the wire, when silicone insulated wires get hot they soften, may sag then if something is close enough make a ground/arch. Especially if its above the exhaust manifold.

I remember for low idle speed problem an AMC manual was suggesting for checking spark for each cylinder, so probably this should mean idle on a v8 can not be rough enough to notice w/o one cylinder.