Intermittent Misfire – 1991 Explorer 4.0L OHV

[Nate]

Hey all, big fan of ETCG here, finally decided to register on the forums and ask a question that’s been bugging me. Just a warning.. there’s a lot of info to take in. Thanks in advance for reading.

History;
Got the Explorer about a year and a half ago, it was in rough shape. I bought it to learn how to work on cars and hopefully have a reliable winter 4×4 vehicle. Last year, it started leaking oil, scratch that… POURING oil out the back. Looking at the options, I figured the best solution was to pull the engine and do a few things. Fixed the leak, it was the oil pan gasket. Popped a new rear main seal in at the same time. New soft plugs, timing chain & sprockets, water pump & timing cover gaskets, front crankshaft seal, engine mounts, and radiator & heater hoses. Got the engine back in and it runs great.

Fast forward a few hundred miles and I’ve got a random misfire. Yea, I know this is a tough one and the possibilities are endless… It’s been almost 700 since pulling the engine, but this problem started 1-200 miles ago.

Here’s what I know;
– This misfire, I thought it was only a hot idle issue but now I’m hearing it as the engine’s warming up, before the thermostat opens. There’s no lack of power, it accelerates nicely and drives smoothly on the highway. No bogging or hesitation, winds up to 4,500RPM without complaint. Does pretty good considering it’s 24 years old with 230,000 miles. The misfire seems to be really bad if I stop the engine and restart it a few minutes later. I did this the other day when going into the Post Office, I felt like it was going to die on me trying to leave the parking lot. This was the only time it misfired while driving too, not just at idle. It cleared up with more RPM.
– Spark plugs & wires have less than 5,000 miles on them. I bought the premium wires and Autolite copper plugs. I’ve pulled the plugs twice since this issue has shown up, all plugs look identical and normal. Maybe a little too white but I’ve never seen a nice light tan on plugs from this vehicle.
– Coil pack tests good both hot and cold, right in spec with primary & secondary winding resistance. The connector going to the coil pack looks kind of shady but this didn’t affect the several thousand miles I put on it before this issue.
– Lower intake manifold leaks coolant slightly. I’ll get around to replacing it but I’m halfway wondering if this is the cause, maybe it’s sucking in air into the intake tracts as well.
– None of the fluids are contaminated. ATF is red, oil is brown, coolant is green.
– Driver’s side head was replaced 10+ years ago by previous owner who’s a competent mechanic.
– When I pulled the engine, I unexpectedly had to remove valve covers and didn’t have gaskets. Right now they have Permatex Black on them.
– Injectors are original, with almost 230,000 miles. Listening to them with a stethescope, they don’t miss a ‘tick’ and sound even, even when I can hear the engine misfiring in the background. They’re the older single-pintle style, I’ve been thinking about upgrading them to the style they used in the 1993 Ford 4.0L OHV.
– Valvetrain ticks when cold, goes away when warm. Previous owner replaced pushrods & rocker arms, said it fixed the issue. Then the vehicle was given to his son for several years and a lack of oil changes probably wore the valvetrain down again. I have replacement rocker arms, just need to buy the pushrods and gaskets.
– Tachometer has dropped out 2-3 times since removing the engine. I don’t know if this is related to the misfire but I’ve seen the tach drop to 0 for 2-3 seconds then go back up where it’s supposed to be. Happens rarely. Doesn’t happen while the misfiring is going on.
– No check engine lights, not even during heavy misfiring.
– MPG has always been somewhat disappointing. It was pretty low when I first got it, due to the previous owner disconnecting the SPOUT connector due to engine pinging. I found a new one and have been using premium ethanol-free gas with no pinging. MPG was 14.275 over 1,150 miles last year. I don’t expect Prius-style MPG but I’ve read about people getting 18-20.
– Vacuum lines were all replaced with silicone hose. I had several cracks in the plastic lines and after seeing replacement rubber boots fail within a month or two, I just ripped it all out and went silicone.
– Fuel pump was replaced by previous owner, probably 10+ years ago. I checked the fuel pressure last year and with the engine idling, I was at 30PSI. Key on, engine off pressure was 35PSI. Seems kind of on the low side but there’s no issue with heavy acceleration or highway driving.
– Fuel pressure regulator has never been replaced but doesn’t leak from the vacuum line. I have a replacement ready, for when I take apart the lower intake manifold to fix the gasket.
– Fuel filter was replaced by me, less than 3,000 miles ago.
– It seems like the fuel rail empties. Sometimes I’ll have to tap the gas pedal while starting. I’m not sure if this is leaky injectors or if the check valve in the fuel pump is starting to go (which might be causing lowish fuel pressure?).
– I replaced the O2 sensor not long after I got it. It had the factory one in there, I figured it was good preventative maintenance to go ahead and do it. Purchased a NOS OEM Motorcraft unit.
– Battery cables are new and many if not all of the connections for sensors have di-electric grease on them either from me or the previous owner.
– MAF & Throttle body are cleaned. Intake Air Temp sensor and Engine Coolant Sensor were replaced but with aftermarket units. IAT had failed, ECT was replaced for preventative maintenance.
– Idle Air Control body was replaced with another unit in an attempt to diagnose.
– Compression test on 9-20-2014. I’m not sure if those cylinder numbers are accurate. I performed the test with all spark plugs removed but I forgot to open the throttle. Test was performed cold on dry cylinders.
Cylinder 1 – Test 1: 190-195, Test 2: 195
Cylinder 2 – Test 1: 190-195, Test 2: 190-195
Cylinder 3 – Test 1: 190-195, Test 2: 195
Cylinder 4 – Test 1: 175, Test 2: 165-170
Cylinder 5 – Test 1: 185-190, Test 2: 185
Cylinder 6 – Test1: 180-185, Test 2: 185

What I haven’t done but probably should;
– Check resistance on ECT & IAT sensors. (Admittedly, could be bad. Might have been a poor choice to go with aftermarket sensors.)
– Check resistance on plug wires. (Wires really should be good, just a few thousand miles old.)
– Perform KOEO and KOER tests. (I don’t expect much here since the injectors are batch fire and the computer cannot perform a cylinder balance test.)

I think that’s all I know for now. If I think up something else, I’ll edit my post but I’m hoping this is enough info to get pointed in the right direction. If you’ve read this far, thank you for your time 🙂

Topic

[college man]

Try doing a power balance test with fuel and spark to see what cylinder is down on power.
I’m thinking # 4 with the low compression.

http://www.ericthecarguy.com/faq/solving-automotive-performance-issues

[Nate]

Pretty sure I figured out the issue, or at least the major one. First though, doing a balance test on this engine is awkward. The coil pack fires in pairs so if you disable one, you’ve disabled 2 and on this V6, that’s 1/3 of the engine’s power. Accessing the injectors on the driver’s side is pretty impossible but the passenger side is doable. I guess a guy could pull a plug wire and then ground it to the engine to just bypass one cylinder but that doesn’t seem exactly ideal.

Anyway, the misfiring got so bad the other day that I could feel it in the floor while cruising down the highway. At that point, I had enough and when I got home, I pulled all the plugs with the engine warm. I was hoping to see a leaking injector or some fouling on a plug or… something. All the plugs looked identical at first glance. I’m not sure what made me think of it but I decided to test the spark plug wires via ohms test. #6 wire had absolutely no continuity on any ohms setting. Bingo. I put on one of the old ones in it’s place and have driven it the last 2 days, it’s running much better.

There’s still an occasional slightly rough idle but I did end up doing a KOER test and it came back with an EEC-IV code 13 for ISC/IAC issues. It’s got the original IAC motor/valve on it with 230,000 miles and even though I cleaned it, it’s probably due to be changed.

The thing that irritates me is that I purposefully bought premium spark plug wires, Belden from NAPA, and installed them less than a year ago. Because of this, I didn’t suspect them. I was hoping by stepping up and getting quality parts that I wouldn’t have to worry about them. Oh well, least I got a lifetime warranty. Lesson learned too, check the simple things first.

[college man]

Glad its working out.

[Author]

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