stumbling under load , slight backfire ?

[Guest Mr.Pathfinder]

I have a problem with my 1991 2wd Pathfinder . It has an erratic idle that gets worse under load ( when the a/c is on ) I have searched and read other post but I hope my description may lead to a more specific diagnosis .When I accelerate hard the engine starts popping like a slight backfire sound and I have noticed black smoke puffing along with the pops , not a lot of smoke just little puffs .The popping is rythmic and increases with the rpm .

One strange thing the previous owner had a wire spliced into one of the tiny wires coming off the a/c compressor ,this wire came off and I was unable to determine where it went . I have only had the truck a few weeks and this started only this week , they may be related ?

 

Thanks !

[Guest Mr.Pathfinder]

I disconnected my O2 sensor and it ran great for about 10 miles ( I turned it off to go in a store ) on the way home it started doing it again :confused:

[ChromoTech]

Do a full tune up

plugs, wires, rotor, distributer cap, flush out the throttle body with carb cleaner, check out the sensors and O2 sensors

[ChromoTech]

Does it feel like its jerking when you accelerate?

does it sound like an exhaust leak (airy repetitive ticking sound coming from one of the manifolds)

[Guest Mr.Pathfinder]

It feels like it is cutting out ,My first thought was the coil wasnt working correctly .It gets a lot worse under full throttle . the cap and wires look recent as does the filter but I will look at things a little closer. No exhaust leaks .

I wonder why it ran so well with the o2 sensor unpluged and then went back to bad ?

[Diesel Boy]

How are you for mechanical competence??

Well, as you would do, start with the electrical, then go fuel, then mechanical.

So to me it sounds like you a have a cylinder that is not firing at all.

Go around and pull each plug leed of one at a time.

With the engine running, pull a leed off, listen to hear if the missfire gets worse, meaning you have two cylinders now not firing, then put the lead back on and do the next one and so on.

If im correct (which is hard with out actualy being there to see what the vehicles doing in person) You should adventually get to a leed that when pulled makes no effect to the way the engine runs.

You now have a place to focus on. You can either remove that spark plug from the offending cylinder, or leave it there and get another and insert in into the end of the leed. Rest the spark plug against the cylinder head and don't touch them. Restart the engine and look to see if there is nice fat blue spark. If not start replacing rotors, leads and plugs, or swap leads from another cylinder with the off ending one and see if it makes the plug spark. Then you know wether the leeds good or not.

If you get a spark with the existing lead, and havn't pulled the sparkplug from the head, then do that, see if the origional plug sparks. If it doesn't replace the plug and it should be fine.

If it sparks, then the problems most likely not electrical.

If you have EFI run the codes on your computer. Most likely not fuel system related as the puff of smoke and back fireing indicates there is fuel reaching the combustion chamber, just not being burnt in there, exploding in the hot manifold instead.

Your next best bet is to monitor your coolent usage, either gaining or loosing water, looking to see if there is an oily sheen on top of the water under the radiator cap, and in the header tank. Also, when the engine is at normal opperating temp, pull the radiator cap off and look for a steady stream of air bubbles coming up through the water.

Also keep an eye on the condition and amount of oil in the sump.

Is the oil grey or milky looking, and does the level increase. Goes hand in hand with loosing water from the cooling system.

So where am i going with this last bit???

Im thinking a leaking cylinder head gasket.

If you have the money, get a compression check done, or do it the cheap way, dissconnect the high tention lead between the coil and the distributor. Turn the motor overon the starter for a few seconds listening for iregularties in the speed the motor turns over. You should be able to hear each compression, nice and evenly, with no difference between them,. If while turning the engine over it reaches a compression where it kind of skips, or speeds up, then drops back again and you can count 1 2 3 4 5 skip 1 2 3 4 5 skip, them you may save your self time messing around and go straight to a mechanic!!!

Just some thoughts,

Peter.