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Cylinder four not firing
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Elijah Zane
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PostPosted: Wed Feb 01, 2023 3:27 pm    Post subject: Cylinder four not firing Reply with quote

I have recently purchased a fuel injected 78 bus. I did a lot of work on the engine replaced the rings and lapped the valves. I got it all back in a seemed to run well. I took it to somebody and they immediately said that a cylinder wasn’t firing. We narrowed it down to cylinder four. I know it’s got a spark and has good compression. I’ve replaced the fuel injector but it still won’t spray while cylinder three is. It’s not the wiring because we swapped number three and four fuel injection wires and it was still four. I’m really not sure what it could be now. I replaced all the fuel lines, pressure regulator, and fuel pump. Does anyone have suggestions? I think this is an issue the PO dealt with and politely didn’t tell me so it’s been going on before I did work on the engine.
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busdaddy
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PostPosted: Wed Feb 01, 2023 3:53 pm    Post subject: Re: Cylinder four not firing Reply with quote

Welcome!
Good job on describing what you've tried so far!, have you physically removed the injector and swapped it?, or tried putting it in a baggie to see if it makes fuel?
The wire swap test is pretty conclusive usually, but you never know, where did the injector you replaced it with come from?

Tried a different spark plug?, sometimes they will spark in free air, but lose continuity under compression, maybe try swapping it to a different cylinder?
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Elijah Zane
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PostPosted: Wed Feb 01, 2023 4:26 pm    Post subject: Re: Cylinder four not firing Reply with quote

Thanks! Yes I actually took out number three and four fuel injectors and watched for spray. Number three worked number four didn’t so I thought I had it figured out. Then I swapped number four fuel injector out for an old one, I replaced all four injectors, and still no fuel came out while fuel injector three would spray. I have a receipt from the previous owner that had some recent work done and they swapped a fuel injector out too so I would assume they were having trouble with number four cylinder as well. They had also recently replaced the spark plugs and you can see that one cylinder wasn’t working but unfortunately I didn’t label where they came out of.
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Elijah Zane
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PostPosted: Wed Feb 01, 2023 4:40 pm    Post subject: Re: Cylinder four not firing Reply with quote

I haven’t tried this exactly but I did put an old injector on cylinder four that I know works and took the wiring from cylinder three that I know works and tested to see if the injector would spray and there’s still nothing spraying from cylinder four
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PostPosted: Wed Feb 01, 2023 5:04 pm    Post subject: Re: Cylinder four not firing Reply with quote

Well that's really odd...... Confused

My next step would be to look over the fuel rail for a blockage, and the short hose between it and the injector, sometimes a flap of rubber gets cut loose pushing it on crooked and blocks things. Aside from that I'm almost stumped, if #3's wiring fires #3 why the hell won't it trigger #4?. It's batch fire so order doesn't matter, the resistor pack's circuits are cylinder specific, but if you swapped wires that should rule one being bad out.

Last thing to consider is a huge vacuum leak, missing or poorly seated injector base seal?, upper cylinder tin pinched under the manifold gasket?, phenolic gasket spacer cracked?......
But if you've proven the injector isn't making fuel then it's upstream from there.
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PostPosted: Wed Feb 01, 2023 6:08 pm    Post subject: Re: Cylinder four not firing Reply with quote

Did you check the wire plug to #4?
If both injectors are failing to open/pass fuel, it could be the final link, right there at the injector prongs.
Had a Nissan engine, #1 injector needed trimming of the plastic body to properly seat the wiring harness/control wires. fun.
Also check the wires, they can back out & away, failing to complete circuit, even though the harness looks properly seated.
Haven't seen that on a classic VW Bosch injected bus, but it crops up with older Asian cars fitted with aftermarket mass-produced injectors.
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PostPosted: Wed Feb 01, 2023 6:11 pm    Post subject: Re: Cylinder four not firing Reply with quote

I’ve checked all those things fairly well and I replaced all the gaskets when rebuilding the engine. I just removed the metal tubing to those two injections and unfortunately there was nothing blocking number four.
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PostPosted: Wed Feb 01, 2023 6:22 pm    Post subject: Re: Cylinder four not firing Reply with quote

timvw7476 wrote:
Did you check the wire plug to #4?
If both injectors are failing to open/pass fuel, it could be the final link, right there at the injector prongs.
Had a Nissan engine, #1 injector needed trimming of the plastic body to properly seat the wiring harness/control wires. fun.
Also check the wires, they can back out & away, failing to complete circuit, even though the harness looks properly seated.
Haven't seen that on a classic VW Bosch injected bus, but it crops up with older Asian cars fitted with aftermarket mass-produced injectors.


The issue I’m running into if it is a wiring problem is I’ll take a wire I know works on a fuel injector on that same side and it will spray. I’ve tried multiple fuel injectors that I know work on number four and now suddenly they don’t work. Could it be a fuel pressure issue?
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PostPosted: Wed Feb 01, 2023 7:31 pm    Post subject: Re: Cylinder four not firing Reply with quote

Elijah Zane wrote:
Could it be a fuel pressure issue?

A simple water pressure gauge from Home depot that reads 0-50 PSI will answer that, get a hose nipple that fits 5/16" hose while you are there.

Is this happening in the driveway?, or during road testing as well?
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PostPosted: Wed Feb 01, 2023 7:45 pm    Post subject: Re: Cylinder four not firing Reply with quote

Is this happening in the driveway?, or during road testing as well?[/quote]

I’ll try a pressure test tomorrow do you know what fuel pressure I am aiming for? I also realized I installed fuel hose that says it should not be used on fuel injected systems could that potentially cause it?

It happens when I road test it as well. I probably drove it a total of 35 miles and attempted to break in my new rings. I thought it was running great until I figured out cylinder 4 was never firing. I say that because cylinder four spark plug still looks just about new.
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PostPosted: Wed Feb 01, 2023 9:21 pm    Post subject: Re: Cylinder four not firing Reply with quote

Non FI hose is usually labelled like that due to pressure limits, L-jet averages 25-35 PSI so it's not anything like some modern systems. But you still should invest in hose that's rated SAE J30-R9 or higher, it's FI rated and stands up to Ethanol so you'll get a few years between hose worries, cheap out and you risk a Car-B-que in any period past 6 months.

Fuel pressure should be ~ 26-28 idling and 32-35 under power (or with the vacuum hose pulled off the regulator).
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PostPosted: Wed Feb 01, 2023 10:24 pm    Post subject: Re: Cylinder four not firing Reply with quote

I would check two things. First, are you sure that the 3-4 fuel rail isn't clogged just where the #4 spigot is? Also make sure that your resistor block is not damaged.

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PostPosted: Thu Feb 02, 2023 1:11 am    Post subject: Re: Cylinder four not firing Reply with quote

Smoke it



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PostPosted: Thu Feb 02, 2023 9:32 am    Post subject: Re: Cylinder four not firing Reply with quote

did you put a noid light on #4? sounds like maybe a ballast resistor lead is broken.
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PostPosted: Thu Feb 02, 2023 10:33 am    Post subject: Re: Cylinder four not firing Reply with quote

grasping at straws for me - and my bus is elsewhere...... so I can't look.

What about something in the little test port in the fuel rail there. I don't remember if that piece of fuel rail is upstream, or downstream or mid way along that fuel path. But I think it is between #3 and #4. Perhaps someone lost the cap for the test port and screwed in something in that blocks the fuel delivery? Or crap just got in there.
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PostPosted: Thu Feb 02, 2023 10:42 am    Post subject: Re: Cylinder four not firing Reply with quote

just put a noid light on it and see if it is getting a signal. They are easy to buy and quite inexpensive.
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PostPosted: Thu Feb 02, 2023 1:35 pm    Post subject: Re: Cylinder four not firing Reply with quote

Check the four wires coming out of the resister pack (left side on fire wall).
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PostPosted: Thu Feb 02, 2023 7:03 pm    Post subject: Re: Cylinder four not firing Reply with quote

Yes, remove the fuel rail test port plug and see what you find
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PostPosted: Thu Feb 02, 2023 7:04 pm    Post subject: Re: Cylinder four not firing Reply with quote

but if you screw up the aluminum washer when you do, it is a PITA to locate replacements. I had to buy a bag of them to get one.
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PostPosted: Thu Feb 02, 2023 7:17 pm    Post subject: Re: Cylinder four not firing Reply with quote

Hmmmmm... if the test port was somehow obstructing the rail then cyl #1 & #2 would also be starved for fuel, and the fuel pressure would be off the charts:

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