I posted this in the large truck forum nut thought would get more attention here...sorry for the double post!

Hi guys, need a little help here. My 91 F700 has a f.i. 429 that seems to have a cylinder not firing only during rapid acceleration in neutral and when accelerating under load. When in neutral I press the go pedal slowly it is fine, if I press it a little faster I can hear and feel the "miss". Exhaust smells pretty rich when I do this. My driver tells me he feels it has lost about 20% power when driving, when he drives away I can hear it as well.

New plugs, wires, cap, rotor, O2 sensor, timing has not changed and is set right. I put a new egr valve and still has this problem. It was running really good for a few months with all the new stuff mentioned above and then one day just started to run like crap. Fuel pump was replaced 1 year ago. I pulled the plugs on the driver side and they were all nice and tan, haven't pulled the other side yet.

My question is where do I start with diagnosing this problem? I was thinking maybe a bad fuel injector but if it wasn't spraying why would the exhaust smell so rich? Thanks for any insight you might have. I wish this thing had an OBD port!

Derek

 

Are you absolutely sure there isn't any burn-through spots cause by the exhaust on the spark plug wires?

Are these plug wires a good quality product? There could be arching from wire to wire if they lay on each other.

Making sure your injectors are working, they should click. You can feel the click of the injector with your finger tip.

I am assuming the 429 FI is very similar to the 460 setup, I have not seen one before in a 1991. Learn something new everyday. I worked on a GM sedan a while back encountering the same miss description you have, but 2 plug wires were misplaced. Take a spray bottle with soapy water and spray the wires while the engine is idling to see if there may be arching (shutting shop lights off of course).

 

New plug wires, firing order verified, I also just put new heat shields on the manifolds because, as you pointed out, the wires were getting fried. I will check for leaking spark at night and will see if I can feel the injectors. Thank you for the input.

 

I know I stated the plug wires were new and that I had new oem heat shields but I didn't have the pass. side on yet. Didn't think or see the plug wires, being only 2 months old, would have fried yet.......Thank you for the help!

Posted in the large truck forum.........


Alright, found and fixed the problem! The set of plug wires were about 2 months old, but without the heat shield on the pass. side one of the boots was completely fried. I had been trying to get the heat shields for quite some time as I kept burning wires every 6 months or so. I finally was able to find new shields and at this point had only put on the drivers side. The new exhaust manifolds were cast from what seemed a larger mold and were sticking way out past the ports on the heads to the point where there was just enough room to get the plug boot in there and some of them touched the manifold resulting in them getting fried. In addition to having both heat shields on now I used a carbide burr set to bring down the "overpour" on the castings. Hopefully now these plug wires will last more than 6 months.
Fuel filter had some dirty cr@p in it so I changed it as well. Thanks for the help everyone! Sometimes the problem is right in your face and you just don't see it!


Here is a pic of what I tried before I could get the oem heat shields.......didn't work!







I will get a "now" pic of the oem shields installed.