Missing Under Load '78 Westy 2L FI

[Jivermo]

Need some help. Yesterday, going uphill in the Skyline Drive, in third, I began to experience an intermittent miss. Slight loss of power with the miss, but it went away on level straightaways. This morning, going along 77 in North Carolina, I'm getting the same thing. I can cruise at 65, but when I'm going up a grade, I'm getting this annoying miss. I'm at a friends in Charlotte now, and am starting to check things out. Any suggestions will be welcomed!

[Amskeptic]

Any suggestions will be welcomed!

Perform a Next Morning Pre-emptive Tune Up with the following forensic points in mind:

a) valve adjustment - look for tight valves
b) if equipped, set points - look for tight gap before messing with it
c) check air filter, if sunlight readily passes through it, it's fine
d) replace fuel filter - cut open old one look for rust
e) clean/gap plugs and make sure all spark plug wires do not cross each other
f) based on plug color, do you need to adjust mixture?
white richen up
tan OK
black lean out a bit
(because you have been on the road, they will be biased towards lean which is OK but no white outer ring allowed)

Intermittent skips means all grounds including double relay ground, and check white wire from ECU to +1 (-) on coil. If you can slop it around even a little on the terminal, gently crush so it slides on with resistance to the stop
Colin

[Jivermo]

Well, got down here to lower South Carolina, and I'm kinda stuck. I had to get off 95 because the missing got worse. I can drive OK at about 40 or so, but getting up to it is a bear. I replaced the points, new fuel filter, plugs look OK, air filter good, checked connections on double relay-does that go bad gradually or all at once? Fuel pump? Checked all vacuum hose connections. Should I buy some new plug wires in case these are shot? They have a lot of miles on them. I'll readjust all the valves, also. Any other thoughts?

[Psucamper]

I had similar issues on my 78 FI bus. Depending on whose plug wires you are using (mine are Bosch) the phenolic plugs at the end have a built in resistor. Think it's a couple of k ohms. In desperation I measured the resistances and one turned out to be several megohms. New wires and plug ends fixed it. Also, the rotor in the distributor also has a built in resistor that can go south. Messes up the sparking.

[Jivermo]

I replaced the rotor. The wires are old Bosch. I'm thinking to get some universal plug wires at an auto parts here tomorrow, If I can find one, and make them up to fit. Thank you for your help.

[Amskeptic]

I replaced the rotor. The wires are old Bosch. I'm thinking to get some universal plug wires at an auto parts here tomorrow, If I can find one, and make them up to fit. Thank you for your help.

We cannot just throw crap at the wall and hope it sticks. Do you have the ability to ohm-out your spark plug connectors? If they are at 1,000 ohms, they are good.

You cannot just push on. If the engine can't get you up past 40 mph, DO NOT DRIVE IT. Diagnose and repair!

Use your senses! You have a co-driver? Let them drive while you sit in the back seat with the access lid off.
Lightly assist the wiper towards rich, then lean while it is misbehaving. Let's say you help reduce symptoms towards rich and make it worse towards lean.
Do not short-circuit your diagnosis with a fast conclusion "oh, it must be too lean". No. Make only a note that it is fuel related, i.e. starvation. Then you can focus, does the fuel pump get intermittent when hot?

Let's say there is no difference at all when you do the wiper test, but the car is misbehaving steadily.
Get out your timing light, not to time it so much as to look at the consistency of the sparks. Put the inductive pick-up on the center coil wire. Get out on the road with the timing light accessible from the back seat, if/when the engine starts to miss, shoot the light anywhere, you just want to see if the spark flashes happen to disappear when the engine bucks.

Be creative, thoughtful not panicky and keep trying to discern, is this fuel or spark? What steps can lead you toward a solution? Be as happy when you can determine what is NOT a problem as when you determine what IS a problem. That helps you narrow down variables. Fellow forum member who participate here, do not throw ideas too willy-nilly. Let's be scientific and focused.
Colin

[Jivermo]

Thanks, Colin. Here is what has occured: i found that the blower duct had come loose from the heat exchanger, and reseated that yesterday-right after I had replaced the plug wires. Everything else you had suggested in your first post I had done. When I started it up, it ran great, and I drive 250 miles into southern Georgia with it running well. I drove over a rough dirt road to get to the place where i stayed. This morning, I got up and started to go, and the problem is back. Starts right up, you can rev it in neutral, but when I start up, under load, it breaks down. Take my foot off the gas and it will return to a good idle. I rechecked the blower duct, and it is seated well. Would that have contributed to the symptoms I described? Coil gets pretty hot, by the way. I don't have a timing light with me.

[aopisa]

I just went through a month of chasing down a similar problem. Bus would idle fine, but buck and miss under load. It would come and go. However, it progressively got worse until it was undriveable.

I believe it was Colin who said, "Grounds, grounds, grounds!"

Start with the negative battery cable. That was the one I glossed over and caused myself much woe. Check, clean thoroughly and tighten. Also, the FI grounds on the plenum, the double relay ground, and the transmission ground strap. The FI system really does not like an inconsistent signal caused by a bad ground. It is an easy and free thing to check.

Better than just replacing parts. Ask me how I know.

[Amskeptic]

I just went through a month of chasing down a similar problem. Bus would idle fine, but buck and miss under load. It would come and go. However, it progressively got worse until it was undriveable.

I believe it was Colin who said, "Grounds, grounds, grounds!"

Start with the negative battery cable. That was the one I glossed over and caused myself much woe. Check, clean thoroughly and tighten. Also, the FI grounds on the plenum, the double relay ground, and the transmission ground strap. The FI system really does not like an inconsistent signal caused by a bad ground. It is an easy and free thing to check.

Better than just replacing parts. Ask me how I know.

The Voice Of Experience is a beautiful thing. It is hard-won and humble.
Colin

[Jivermo]

OK...well, I got back to Miami after much effort, with this on again, off-again problem. I spent time today pressure testing my fuel pump-it appears all right, going over all the grounds, making certain that the contacts in the double relay white boxes and the 7 pin AFM connector are indeed seated well, and taking the S boot out to check for cracks. Things are pointing towards an ignition problem. If the spark is cutting out, where should I be looking for the problem?

[Amskeptic]

If the spark is cutting out, where should I be looking for the problem?

Is the spark cutting out? You can see it stop flashing the timing light under this bucking?
Colin
(hot coil may have damaged winding insulation. Decent enough cold, a misfiring mess when warm. Check resistance before you replace it. Hot misbehavior occurs with starter solenoid and fuel pump windings, too. Temperature independent symptoms tend to be more about connections)