IBWiglin

So I have a '91 Xtra Cab with 4WD and a 3VZE motor. Just took it in for emissions and it failed pretty miserably for HC emissions at idle. Here are the test results:
At Idle:
RPM - 1032
CO2 - 14.57%
O2 - 0.31%
HC - 591 PPM
CO - 0.181%

At 2500 RPM:
RPM - 2472
CO2 - 15.01%
O2 - 0.24%
HC - 32 PPM
CO - 0.113%

I don't know a ton about using all the numbers to figure out exactly what's happening, so I'm looking for a little help. I'm pretty sure that I have a soft misfire at idle. I've experienced a dead misfire on this vehicle before (a fuel injector plug came loose) and this feels much smoother, but there is definitely something amiss (pun intended).

The intake hose was cracked, so I replaced that, but still have the high idle and puttering below 2000RPM. I'm going to pull the spark plugs today to try to figure out which cylinder is causing the issue. I only had a few minutes the other day so I pulled the plug on cyl. 6 and it looked okay. The distributor cap, rotor, plugs and wires were all replaced 10k miles ago or so. It seems like a clogged fuel filter is unlikely since the miss is at idle and not when the engine needs fuel.

I did advance the timing 1-2 degrees past spec to improve power on the highway. But I did that 6-8 months ago, and didn't have any problems with a high idle or pinging until the last month. When I take a look today, I'll try setting the timing back to spec and see if that affects the idle at all.

I'd love to hear any thoughts or suggestions! Thanks in advance.

Yotard

You left the nox numbers out. Need them to run on the calculator. You need to set your timing back to stock as that is a requirement when they check smog. It would be a good idea to do a tune up. Plugs wires cap and rotor air filter fuel filter if it's been a while. I usually seafoam first before doing any of that. I use the fog type through the intake. Your HC at idle is really high meaning fuel isn't being burnt completely.

IBWiglin

Yotard, I did a tuneup relatively recently (17k miles ago). About the NOx, I live in Utah, and apparently they don't measure NOx emissions...there isn't a category for it in the printout that they gave me.

So I pulled the plugs, and they looked reasonable, but the number 6 plug was a little carbon fouled. I went ahead and replaced them just to be safe. Then I set the timing back to spec with a timing light. Still no change in the high idle or the puttering at idle. It seems like the next step is doing a compression test, with a keen eye on cylinder #6?

There's one other thing I noticed. When I had the timing light on, there was a noticeable intermittent flicker in the strobe. About once or twice each second, it seemed like the strobe didn't light up when it should have. I put the inductive pickup on a different spark plug wire, and the issue remained. Then I tried putting it on the wire from the ignition coil to the distributor. There was still an intermittent flicker. I've never noticed this before, but I guess it's possible that it's just a problem with the timing light. This made me suspicious that the ignition coil could be on its way out. However, that seems like a problem that should get worse at higher RPMs, and not better. Any thoughts?

scope103

You could have a problem with the distributor. Here is the FSM: http://web.archive.org/web/201212070...12onvehicl.pdf
DO try to wiggle the rotor with the cap off. If you have a bad bearing in the distributor that COULD cause intermittent firing. DO check the plug gap. DO carefully move the plug leads at idle, to see if the miss changes.

A compression check is a good thing (and not that hard to do), but your high HC is most likely due to a misfire, and I would look closely at ignition for that.

In California, we have to have the timing dead on to pass smog. I understand other places (Utah?) don't care as much.

IBWiglin

scope - thanks for the advice. I've been busy the last couple days, but I finally got some time to do some snooping this evening.

The spark plugs have been gapped properly. I didn't test the TPS, but the ECU responds to jumping E1 and TE1 by adjusting the timing advance, so it can't be that far off.

I pulled off the distributor cap and checked the resistance of the wires. All was OK. The air gap was fine. There was a tiny bit of play in the dizzy shaft, but I couldn't get the air gap out of spec while pressing the shaft in one direction or another. The ignition coil tested at 0.8 ohms, while spec is 0.5-0.6 ohms. However, the engine was warm when I tested it and the manual says to check it when cold. I'll check it again tomorrow.

The most significant thing I found was this. Pulling the cylinder 6 spark plug wire at the dizzy seemed to have no effect on the idle...

I'm planning on doing a compression test next. I'll report back with results

IBWiglin

So I did a compression test today. I didn't test all the cylinders, only 2, 4, and 6. Cylinders 2 and 4 checked in at ~125psi. Cylinder 6 came in around 70 psi.

It seems like the most likely issue is a burnt exhaust valve. I'm a little torn about how to proceed. The truck has 240k miles, but the engine (supposedly) has only 150k. The previous owner told me the engine was replaced under warranty at 90k. I'm currently considering four options:
1. Fix the engine right: replace both head gaskets, pull the oil pan, replace the piston rings and hone the cylinders, have the heads cleaned and milled, have valve seats replaced as necessary, lap the valves, replace all seals in the heads.
2. Do the absolute minimum: pull the one head and fix the minimum amount necessary to get the truck running right.
3. Sell the truck as is.
4. Find a 5VZ-FE and put it in the truck.

I'd like to hear from anyone who's had issues with 3vze's and what they did about them? Do you have any advice?