75 FI Bus Engine Misses- Need Help

[stony]

Hallo everyone, we made it down the Oregon coast, and the last two days we didn't have one drop of rain (wow....)

Since the rain stopped, the engine misses out every once in a while. There is no regularity it seems. Suddenly, for a split second, the bus kind of stops or brakes, everything shakes and then it is good again. We have had that same thing a couple months ago, somewhere in Wyoming. Back then we checked valves, timing and dwell again and changed the fuel filter. It seemed to have done the trick. I rechecked the timing this morning. A fuel filter change (after 2000 miles) is the next thing tomorrow.

Can that fuel filter clog already? Can a clogged filter cause that kind of engine miss?
If that won't help tomorrow, any other suggestions?
All input is welcome. Thanks a lot,
Stony (and Andres and Luca)

[midatlanticys]

Hi Stony, I'd say another bout with an engine that misses every once in a while after 2000 miles could well be another clogged filter. Is it most prominent at higher speeds/revs when the engine needs more fuel and a clogged filter might not be able to keep up with the demand? Have you ever had the tank out for an inspection and cleaning? Good luck!

[stony]

I rebuilt the engine 3500 miles ago and looked at the fuel tank as well. It looked super clean. The filter that I took out this morning also looked clean from the inside. No particles at all. I will do more driving today, but I doubt that the filter change did anything to fix the problem, since it looked so clean.
Any other ideas?

[midatlanticys]

When you say, "the bus kind of stops or brakes, everything shakes" do you mean like the ignition is momentarily switched off and might be an electrical glitch - like a double relay affecting the FI?

[Amskeptic]

I rebuilt the engine 3500 miles ago and looked at the fuel tank as well. It looked super clean. The filter that I took out this morning also looked clean from the inside. No particles at all. I will do more driving today, but I doubt that the filter change did anything to fix the problem, since it looked so clean.
Any other ideas?

Fuel problems normally are "soft", as in they take away power, they cause stumbles, but they don't just kill the engine entirely, like electrical/ignition problems can.

When I say "kill the engine entirely", please understand that I am not talking about a complete stall, I mean the engine drops out entirely whether or not it comes back in a second.

Electrical supply to the fuel pump is a "soft" electrical problem, because the symptoms are buffered by fuel pressure in the rail as the pump spits and coughs. Electrical supply to the coil will electrically be identical, but the effects on the engine are an immediate crisp bucking or death.

See what I am getting at here? Please try to distinguish between a soft hiccup and a harsh off/on/off/on.

If it is soft, look carefully at the double relay ground wire, the connection at the double relay plugs, particularly the red/blue wire that goes from the relay to the pump, pump connections, and AFM connector, FI ground spades and the white wire from the coil (-) #1 terminal to the ECU.

If it is harsh, look at (+/-) coil connections, points green wire to distributor (dwell too) FI grounds, ignition switch.

Note also, if you are running rich or lean, when the ECU discovers a signal outside its envelope, it plays possum and dies. The whole thing is dead until you get back inside.
Colin

[stony]

Thanks guys,

To me, it is like a harsh hiccup, sometimes more and sometimes less hard. Absolutely random, once even at idle yesterday. I will go and check all the things you told me.
I believe it is more the second, the harder version of a cough.
(New coil didn't change a thing, by the way, and new fuel filter neither.)

I am happy to get replies and ideas, even if I don't answer right away. We are on the road and internet is not always there. Right now we are in Arcadia, CA, heading south in the next few days.

[Amskeptic]

harsh hiccup,
Absolutely random
(New coil didn't change a thing, by the way, and new fuel filter neither.)

I'm leaning towards electrical poor connection. There are many. You need to accept this in the spirit of adventure. Do not throw parts at it. When it happens again, jiggle the ignition switch. It could help cancel the bucking or make it worse or no change. Either way, it helps to narrow down the cause. The cause is in there. It is waiting for you to find it.

[midatlanticys]

When it happens again, jiggle the ignition switch. . . . . It is waiting for you . . . . .

. . . or jiggle the same switch hoping to artificially recreate the cussed hiccup?

[Amskeptic]

When it happens again, jiggle the ignition switch. . . . . It is waiting for you . . . . .

. . . or jiggle the same switch hoping to artificially recreate the cussed hiccup?

Yes, you want to tease the beast. If it should happen only when it is raining, for example, you would get the spritzer bottle out and douse the engine. These annoying intermittent problems are just tempting you to buy parts one at a time, do not succumb.
Colin

[stony]

All right, I'll keep searching. Will go on a wiggling tour...

[Amskeptic]

All right, I'll keep searching. Will go on a wiggling tour...

Update? Or are we in the deep sleep of winter?
Colin

[DurocShark]

Here's a pic of the 78 wiring harness. 75 should be mostly the same...



Your symptoms sound very much like mine. I could run fine for weeks, then get hiccups every time I shifted or revved the engine too high (3k is too high for this).

The tach signal wire is the long gray one in the bottom of the pic. It goes from the coil to the ECU. Mine had a hidden break somewhere. I could pull the harness out of the bus and test every wire and it would be fine. Which sent me on a merry chase for other causes. The Guac was parked for a year because of it.

Now I've spliced a new wire in there and it works great.

[stony]

Haven't found anything yet. I am not sure anymore if it is really a wiring problem. It is absolutely random. Even standing still.

A SF friend told me he had something in his fuel tank that would plug the outlet every once in a while. Then, with the hiccup, when the sucking would be less, that thing would start floating away again. Sounds like a not-so-good thing to fix.
Stony

[Amskeptic]

Haven't found anything yet. I am not sure anymore if it is really a wiring problem. It is absolutely random. Even standing still.

Do not get sucked into anecdotal drift. You could end up chasing all number of leads to their own ends. Absolutely random is extremely unlikely to be fuel related. Crap in the tank outlet would definitely hammer you on the highway under high demand, and recover when demand went down.

Wiring/electrical/ground problems (unless you have certifiably checked each MPC on the entire FI loom + all grounds everywhere + fuel pump wiring clean and tighten spades + a check under the double relay cover for moisture-induced powdering of the contact points + triple-checking the double relay ground + ignition coil #1 FI wire to the ECU MPC where you actually closely inspect each female for correct seating in the plug housing) can give you random drop-dead/instant recovery.
Colin

[stony]

I am pretty sure that the problem is solved (I hope it doesn't come back now that I write about it).

Since I bought the bus, I had always set the timing wrong.
Mr. Muir says to set the timing statically, and that is what I did. 5 degrees ATDC. Now, if I check with the timing light, it is way off, way retarded. Like 12 degrees off. Since I set it with a light, the hiccups seem to be gone. It can be so easy....

Stony