I have the opposite problem to another poster. My #1 cylinder isn't firing. Here's what I've done so far:

1987 Cherokee Laredo, 160,000 miles, good shape, new to me. 4.0L inline size, Renix.

It was running very rough, sometimes no start condition and no power.
-cleaned throttle plate with carb cleaner (it was dirty).
-checked TPS with ohm meter. Very smooth action opening and closing so I think TPS is ok.
This seemed to help with the no start issue but still running rough and loud and smoke out of tail pipe.
-Checked compression-all cylinders between 140 and 150PSI
-Removed spark plug wire one by one. Each cylinder made running worse except for removal of #1 wire. Therefore, issue is with #1
-Pulled wires off distributor one at a time while running. Good spark to spark plug wire (and good jolt to my body each time!)
-Changed #1 spark plug wire (no difference)
-Replaced plugs in #1 with Champion Copper RC9YC (2x) no difference

So, now I'm thinking fuel issue.
-Checked fuel pressure at rail. 32 PSI. Removed vacuum from fuel regulator. Got 40 PSI. That's supposed to be ok.

So, now thinking injector.
-removed wire from each injector while running. Ran rough each time except for #1 (therefore #1 is the issue)
-Checked resistance of #1 injector (got 19ohms) should be ok
-Connected 12 V test lamp across wire connector. Light flashing as engine is running. Therefore, getting some voltage at right time.
-Removed fuel rail and first three injectors
-#1 injector looks ok. Some deposits. Sprayed with carb cleaner.
-Swapped #1 with #2 and put back together.
-Big fuel leak (hadn't attached screws to fuel regulator (I guess you have to do that!!)
-Put all the bolts and screws back on. Started engine.
-Still issue with number 1. Took off spark plugs one by one and expected change if #1 was taken off but no difference.

Running out of ideas. I have a new CPS to install and a new distributor cap and rotor but I find it hard to believe this will solve the problem. I haven't done anything about the C101 connector either.

Anything else to try or thoughts? I took the plug out and it smelled like gas which makes me think it's getting fuel.

 

Wiggle the harness along the valve cover. You have voltage to #1 injector, but is it enough?

 

It's been plugged, unplugged and wiggled and jiggled. It lights the test lamp so it has power.

I'm about to try changing the Crankshaft Position Sensor but can't really see that being the issue. The only thought I have is that it's not firing or injecting at the right time.

 

CPS is not the issue. My point was: Is there enough voltage to actuate the injector? A test of the actual voltage versus the lighting of a testlight might tell you something.

 

Check the connector @ PCM for corrosion.

 

I agree with cruiser54 here. This is almost assuredly NOT a crank sensor problem.

 

Swap #1 plug wire for #6 and see if it moves your problem to #6...or get new cap/rotor/wires.

 

Is there something magical about #1 and #6. I was thinking about doing that too since I think they both are close to TDC at the same time. Is there any chance the fuel injector harnesses could be messed up? How do you know if #1 has the right injector firing?

 

Hard to tell since the power is only on the injector for a short period of time. I think it read 5 volts while running but that could be because it's pulsed off and on so fast.

The injector does click on the bench when you put 12 volts to it.

Weird. I'm starting to think this Cherokee was unloaded on me.

Anyone think it could be a valve issue?

 

Hank's indicating to swap the whole wire #1 wire swapped with #6 wire, as they are approx same length, if the wire is bad then #6 will now have the no fire problem.
I've had wires go bad on a number of vehicles, all ages of wires, new and well used.
Also, look inside your distributor cap, is the terminal clean and intact, did it break off? or heavily oxidized and needs cleaning. Also is the wire making contact at the distributor. Do you have a spare plug you can place in the wire and touch to ground on outside of engine to see if there is spark?

 

Youve got cylinder pressure (140 to 150 psi by your tests), not a valve issue.

 

Sparkwise-I have a new cap and rotor I can put on but when I pull the wires off the cap while it's running I get a nice spark jumping from the distributor pins to the plug wires. Also, when I pull the plug wires off the plugs, I can see the spark jumping to the plugs so I don't think it's the wires. I put a new wire on #1 that I had and there was not difference. I changed the plug on number 1 and no difference (2 different plugs). Gap is .035 and Champion coppers.

Can I switch the #6 injector harness with the #1 harness?

 

Hopefully

I got.

#1-140
#2-145
#3-140
#4-145
#5-145
#6-150

 

Tough one. Do you think the injector is spraying in good condition. I've seen new ones mist like a fine fog, helps the mix and even distribution. Then I've seen worn ones that are very eratic and some times piddle out like a drunken sailer on a fire hydrant, not great for fuel and intake air mix. I guess you did try the swap of injectors between cylinders. The wire arcs from the cap, that's good. I have had a bad pack of champions many years ago, 40% in the package, duds right out of the box or failed within 1st hour of operation, it's frustrating, but it did happen.

 

My understanding is that he swapped injectors from one cylinder to the other and still has the problem.