I have been hunting a missfire in my truck this last week, it causes exhaust pops when decelerating and rough idle, specially at start up, it will still run badly after it's warm, but the RPM won't deviate so badly as it initially does (when the truck is warming up the engine surges and stalls easily). Well, for the first time ever I retrieved the codes of my truck and got these:

KOEO:

565: Canister purge solenoid circuit fault. Given that it's been removed from the truck, it's a no brainer.
628: Excessive converter clutch slippage. This worries me, will keep an eye on my torque converter, but it isn't related to the problem at hand.

KOER:

412: Cannot control RPM during high RPM check. Obviously related to the problem, but gives me no hints.
116: Engine coolant temperature out of range. I probably didn't warm the engine long enough before the test, and my thermostat needs replacing, not a cause.

Apart from that, I replaced the distributor cap (the old rotor is still in there, but it's complete, will replace it anyway), sparkplugs, and wires (need to check the fitting of those), since they were worn enough. Also checked the manifold bolts, but they seemed tight, still a exhaust leak is my main suspect. Under load the engine seems to work alright (maybe not at 100%), this should rule out the injectors being the cause. The truck exhaust pops badly specially when going downhill, decelerating from high revs. There have been no visible flashes at night so far.

So there you have it, any thoughts?.

 

I'd start by getting your thermostat fixed, as that does impact idle control if the engine can't fully warm up.

I'd suspect a stuck/failing IAC valve, or a bad throttle position sensor.

 

Those are my suspects too, but I'll try to diagnose them before blindly replacing. I really thought the codes would help me out. I didn't clean the IAC yet, and it's quite easy, will post info and pics somewhere else when I do it tomorrow.

 

Well they did. The torque converter code should really be ignored until you get the drivability issues sorted first.

 

116 points to an engine coolant temperature sensor fault. This little sensor can cause headaches, so I'd start there. Here is the pinpoint testing for 116 for a 1993 Bronco with a 302. The gist is that you can check the resistance at the sensor to see if it is functioning correctly. Careful if you replace it...I went through three new ones before I found one that wasn't bad out of the box.

You can buy an online subscription to alldatadiy.com for about $30/year. It has the pinpoint testing diagrams for all of the codes. Short of a factory service manual, it comes in handy when troubleshooting.

 

Well, cleaned the IAC to no avail, also installed the new spakplug cables, so my next step will be to test the TPS. I'll eventually check the temp sensor too, but right now I doubt it's the cause of the problem.

 

Try getting the engine as warm as you can, and then running the test. If you still get the ECT code, I'd suspect that.

 

Well, replaced the TPS to no avail (found an original Motorcraft for cheap, so the old one will become a spare). The ECT is clinging for his life to the housing so I haven't been able to remove it, will definitely replace it and retry.

 

Well, replaced the ECT to no avail, still running badly. The ECT was quite nasty so it definitely was a good call to replace it. Just some coolant drips came out when I removed it, so the thermostat definitely needs to be replaced too.

Time to start looking at the fuel System, I'll test the FPR to see if it's broken. To do so I'll turn the engine on and remove the vacuum line to check for a tear in the diaphragm (fuel should come out).

I've noticed the idle is generally bad, but it surges wildly the first time I turn the engine on for the day; the revs just climb and fall, and unless I keep pushing and releasing the throttle the engine stalls. After some warming up the revs become a little more stable and I can stop pushing the pedal. During driving the throttle response is hesitant. Does this point you guys to any condition? I might have gotten bum sparkplug cables, it feels like it.

 

Still getting trouble codes?

 

Haven't re-tested (started raining as I was wrapping things up in the engine bay), but they sure are there.

 

Get that thermostat changed too while you're at it. Generally the engine needs to reach at least 160*F before the computer considers it "warmed up"

 

That's the plan, any other ideas as to what else I could be looking for?.

 

Have you checked the computer for leaking capacitors?

 

No, but codes and operation have been consistent. The trans is working fine too, which leads me to believe the ECM is doing fine. Of course, no harm in giving it a look.