VX bucking bronco

[oh95vx]

Sure wish I'd found this forum BEFORE things started to go wrong with the VX. I live in Ohio where big pickups rule the road, and small gas savers are definitely a minority. Always nice to talk with like-minded folks.

I've had this gal since she was new; she's now at 172,000 miles. For about the last 15,000 or so she's been developing a noticeable miss at steady speeds. That means *any* speed - 25mph, 65mph, anything in between. It's gradually been worsening to the point where the miss has become a buck.

Fuel efficiency has dropped too. I seldom got the EPA 56 highway even when she was new, but in mixed driving I mostly got 46-48. That fell to around 42-44 when the roughness started, and has been more like 38-40 lately.

Until recently she always smoothed out when I accelerated, with a little stumble at the start of acceleration. Lately the engine stays rough under acceleration, too, and the stumble has become a trip-and-almost-fall.

Through all this, the CEL hasn't come on (except when I've been tinkering and deliberately disconnected something). No codes are set by normal driving. Bridging the diagnostic connector produces a steady CEL, no blinks.

Plugs and air filter have about 10-15K on them. I replaced them early in the game, shortly after the first slight roughness appeared; no dice. I put on new plug wires (NAPA, IIRC) about a year ago; again, no change. Have I forgotten anything?

I ran the oxygen sensor tests in this forum and elsewhere. When I spray ether into the air horn the voltage jumps to -1.39 (or therabouts; hard to read) as the engine almost stalls. Pulling an injector plug off sends it to +.42. Both of those seem about right.

The spool valve seems to be working OK. I connected it to B+ at idle and engine speed dropped a bit. (Is that what should happen?)

Checked the TPS with an ohmmeter; I don't recall the exact resistance, but it changed smoothly with throttle position, no jumps or dips (I was hoping for a big glomph at about cruising position, but no such luck).

She runs tolerably well in open-loop mode - during warmup, or if I disconnect something like the oxygen sensor or spool valve. But that probably doesn't mean much.

It sure does feel like she's running 'way, 'way too lean. But why?

Can anyone offer more suggestions for what to try? I'd be most appreciative. Many thanks.

[jpciii]

distributor cap and rotor should be replaced... maybe the distributor itself is going out... have you checked the timing?

[Matt Timion]

jpciii makes good suggestions.

Honestly, if you have 170K+ on that engine and havn't changed the o2 sensor, you REALLY need to. I suspect it may actually have something to do with your problems.

FYI, you can get them brand new on ebay for $170.

[oh95vx]

Originally Posted by jpciii

distributor cap and rotor should be replaced... maybe the distributor itself is going out... have you checked the timing?

Hmm. I had a highway breakdown recently, and the shop they towed me to put in a new ignition control module and a coil. Come to think of it, she seemed to run a little worse after that. I don't think it's usual to pull the distributor when changing the ICM, so I wouldn't think that would change the timing. Still, it's worth a check.

I remember changing the cap and rotor, but I can't think when. Maybe 30-40K miles ago?

Originally Posted by Matt Timion

Honestly, if you have 170K+ on that engine and havn't changed the o2 sensor, you REALLY need to. ... brand new on ebay for $170.

Thanks. That sounds better than the $300+ I was quoted at a couple of local places. I don't see any new ones on Ebay right now though, just several used ones.

The OxS passed the test in this forum (see above). But you still think it's worth trying a new one?

Is there an OxS failure mode in which the OxS tells the ECU that there's less O2 in the exhaust than there really is? That is, could the OxS be lying to the ECU, telling it the mixture is richer than it really is?

If the OxS were flaky, wouldn't the engine run poorly all the time, including at idle? It idles smoothly, though sometimes faster than it used to. (1100 rpm falling to 700-800, compared to the 500-600 it used to idle.)

So many questions!

[GasSavers_Ryland]

when I took my vx to the dealer about a year ago to help me truble shoot my exact same problem, they said they didn't have a reliable way to test the o2 sensor, but I had the same problem you had, only I managed to get 210,000 miles on it befor it died, but about the same number of years, so it makes sense, it seemed as if the problem is that it's fallen out of calibration, and that just happens when it gets as old as it is.
Have you replaced the timing belt and water pump yet? it's a good idea to do every 100,000 miles.

[ELF]

Sounds like a bad valve to me.

[oh95vx]

Thanks for the reminder. I changed the water pump & timing belt at 87,000, and timing belt alone again at somewhere around 160,000 miles.

The bad valve scenario is pretty scary. I really don't think I want to get into pulling the head, and paying a shop to do it is going to be mighty pricey. But if I can find my old compression tester, I'll make 'er breathe into it this weekend.

[SVOboy]

Replacing a valve isn't all that bad, just follow the instructions,

[Bunger]

I would advise a compression check also... if you come back with one low cylinder, get it leak-down tested. Also, check over the EGR system. When doing my swap, I had it hooked up incorrectly so that the valve was always open and the engine ran terribly. (Though that should throw a code for the valve lift sensor.)

[oh95vx]

Just checked the timing, found it a bit retarded, maybe 5-7 deg off. (Hard to see the marks on a rusty, pitted flywheel, had to find TDC with a dowel.)

Still runs lousy.

But #1 plug is black and turns out I had Bosch in there (what was I thinking?). You could just about drive an Expedition through the gap. I'm going to try new plugs, whatever I can get today (probably Champion) and then dig up the NGKs Monday. I'll check compression as I change the plugs.