My '90 Ranger is experiencing a pretty bad misfire after doing a tune-up. I changed the plugs (gapped to .045), wires, distibutor cap, rotor button, air filter, fuel filter, and cleaned the IAC. It had a barely noticeable miss before the tune up, that's why I did it. Now, it's pretty horrible.

At idle, it's definitely noticeable, and at cruising speeds (RPM's below 3K) it bucks and surges. The weird part about it is WOT runs are as smooth and (somewhat) powerful as ever! At idle in neutral it will revv up just fine without any hesitation as well.

First thing I did was check the firing order, again and again. Made sure all my wires were properly seated on the plugs and on the distributor prongs. Pulled the plugs back out, they were all clean and properly gapped. I pulled the wires off one by one to try to isolate the misfire, but every plug wire I removed made around the same impact on the motor.

Next I ran codes. KOEO, I got code 95 - fuel pump secondary circuit failure. Don't know what to do about this. Fuel pumps comes on at the turn of the key, relay clicks... KOER, I got 11 (pass code), so nothing there.

Then I hooked a fuel pressure gauge to the schrader valve on the fuel rail, and at idle pressure was a steady 35 PSI. I also removed the vac. line to the FPR while running to see if gas came out. I checked for vac. leaks as well.

Now I'm outta ideas, not sure what to check now. I've checked everything I know to check, but nothing helps.... Any ideas?

 

on my 4.0 had the same situation. Changed all the ignition parts and nothing. Took it to a shop, they checked everything that I had done, could find nothing wrong. They ran tests said everything was working. Truck has 300000 miles on it. Ws considering getting a reman engine, but since the only thing I had not changed was the injectors thought i would try them first. I had them cleaned at about 100000+ but when I finally put new ones in they fixed everything. The shop had given them a fair rating in their testing. Got mine from autozone about $35 each (they price matched orielly). So if you havent changed them might now be the time to do so.

 

Good trouble shooting on the worse miss after the tune up, but lets think about this some more. After removing plug wires in succession made no noticeable difference, back up & begin to replace one at a time, the rotor, coil wire, distributor cap & if no joy maybe have the coil bench tested for output under load, as those things can affect spark quality on all cylinders.

Was the end to end resistance of the new plug & coil wires in spec????

If no joy, raise the hood at night while idling the engine & look for arcs & sparks, maybe do a wet down test with a spray bottle of water, on each plug wire, all the way back to the distributor, then the coil wire, then coil.

If still no joy, with a stethoscope have a listen to each fuel injector to determine if any sound different than the others.

By then you should have an idea if its a spark or fuel delivery based problem.

More thoughts for consideration, let us know what you find.

 

Ok, so I didn't have a stethoscope so I used the 'ol vacuum hose trick and listened to each injector carefully, and what I found was the #4 injector had a very faint if any clicking, unlike the others who clicked loud and sounded good.

So I decided once again to pull the plug wire with the motor running,,,,, made no difference whatsoever.... Maybe I missed this before??

Used a screwdriver and checked for spark, had good spark, re-installed the wire while running and had no effect on the motor at all. I think I have a bad/clogged injector.

 

Good trouble shooting with your home made stethoscope, sounds like you found a good suspect, keep us posted on how it goes.

 

Got a new injector and got it installed and the fuel rail bolted back in. just have to get the upper intake back together and vac. lines re-installed. Wish me luck!

 

We all have our fingers crossed for ya. How did the resistance of the old injector compare to the others & how did the new one match up???? Its important that their resistances be colsely matched. They should be between 12-16 ohms.

 

Well, it was a no go. Cylinder 4 still isn't firing. I know it's getting a strong spark, but I think it's still not getting fuel.

As far as measuring the resistances, where would I probe to get accurate readings? Is there a way to bench test an injector to see if it's working properly?

 

Disconnct & meaure resistance across the injectors electrial connection.

The injectors should have B+ to their electrical connector at KOEO & the connectors pins/sockets should be clean, bright & tight & no pin/socket, or wiring corrosion or insulation damage.

The fuel injectors are ground switched by the computer, so if electrical connections & wiring to the injector & back to the computer check out ok, you may have a computer ground switching driver problem.

 

As soon as I pull everything back apart, I will measure the resistances and report back.

I'll trace down the wiring as well, look for anything corroded or broken wiring. I know when I pulled the connectors off the injectors they were nice and clean inside, and fit back on tightly (I did not break the locking tabs on the connectors).

 

OK, so I checked the resistances of the injectors, and they all read 15.9 ohms EXCEPT the one I replaced. It reads 17.8 ohms. Now, I'm no electrical genius, but would 2 ohms difference make it either not function properly or fire out of order?

I have 2 injectors that I scored from the junkyard that both read 15.9 ohms, so maybe I should swap one of them in and see what happens.

 

No 2 ohms won't cause the injector not to work, but can cause a difference in that cylinders fuel delivery as the circuit load is different so it'll affect squirt time.

Does the new injector sound like the old one did????

What did the old injectors ohms resistance measure????

 

Old injectors resistance was 15.9 on each one, even the suspected bad one that I replaced.

 

OK, well it doesn't sound like the old injector has an electrical problem, as its resistance matches the others. So since it sounded different, maybe you have a wiring run problem, computer firewall connector pin/socket problem, or that injectors computer ground switching driver has an internal problem, or its chassis ground connection is corrupt.

Keep us posted on what you find.

 

So does that mean I would need a new computer? I've followed the whole injector harness, looked at the big round connector at the firewall, checked all the pins and visually it looked ok, looked at all the grounds I could find, but the problem still persists. I even swapped in another injector with matching resistances. Next thing I know to do is try switching the pcm....