Fuel Injectors

I’ ve been following this sight for awhile now and I had a question and I figured I might as well ask if I ever want an answer. With the fuel pump in the tank the fuel acts as lubrication and coolant, what happens to the injectors if the fuel is shut off and they keep trying to pump fuel. will they overheat or burnout or what?

Hello Tim,

Good question .

It will most likely take someone smarter than I to answer this.

I thought without the fuel going through the injectors they would soon burn out. I made sure I had some replacement injectors available and started driving to see how long they would last running dry. That was four yeas ago and haven’t hurt one yet.

I don’t know how the injectors operate but I suspect the electric pulse opens the injector and the fuel pressure closes it. With the fuel pump dead I think the injector stay open and static.

Hi Tim,
I think I can help with this question. Fuel injectors do not pump fuel at all. From the research I have read they classify as an electromechanical
spring loaded valve. The 30-50 of fuel pressure helps open the injector. 12volt current finishes the job When its time for that injector to fire the computer sends out a voltage to that injector. This is how multi port fuel injection works throttle body injection was a little different controlling fuel flow. So on the wood burning trucks when we turn off the fuel pump the injectors can’t move without the 30-50 psi. Of fuel pressure behind them nothing to wear out. Regards Sean

Thanks for the quick replies

On the TBI on my 93 chevy the injectors open without pressure and if there is sufficient vacuum it will pull gasoline all the way from the fuel tank to the engine. This IS how I run it hybrid when I have to. I also know I can drive that truck without a fuel pump. If I want to be 100% woodgas I have to turn off the injectors. I believe my MFI cars did the same but they are scrapped now as they rotted out underneath and were un-repairable … Wayne has tried to convince me that his trucks do not suck any through the clicking injectors. He did some track testing where he had to run the tank dry so he is pretty sure what is going on with the Dodge trucks. He also ran one of the last ones with the injectors unplugged. That brand and year is pretty much rusted up and gone here so I have no experience with them … Mike

I’ve used fule injectors on diferent applications. Sean is correct they are a spring loaded valve"solinoid". I have put very little pressure agenst them and they still open & close the same. This makes me think the fuel pressure does not affect the opperation. When I gasified my 95 seville I was to paranoid to leave the injectors running so I turned them off. The problem with that was I had to cycle the ignition to get them to come back on. I don’t know if this would be true for other makes.

Ha! Ha! Well folks it turnes out you are all correct.
I worked elbow to elbow for a year with a guy who’s job was to clean, and function flow certify and rebuild as possible automotive gasoline fuel injectors. 3-5 sets a day on an expensive very flexible to set up European test bench.
Auto fuel injectors can be Top, Middle or Bottom fuel fed depending on what the designer wanted. They are needle like pintle tipped or disc orifice. Both types can be single opening or multiple opening spray nozzle. They have design operating fuel pressures from 7-12 PSI to upward of 120 PSI.
LARGE body/low fuel pressure Throttle Body installed fuel injectors I am sure would negative pressure open up and siphon. Think about it. Above the throttle plate they were never designed to be under any more negative pressure than a dirty air filter.
Intake manifold PORT installed fuel injectors had to be able to cleaning shut off the fuel spray even under off throttle coasting and idle to engine stopped high intake manifold conditions.
Some systems especially SMALL electrical coil body type DO use fuel perssure to assist injector snap opening. Others will use fuel pressure to assist in injector quick clean closing.
Like all things in life: depends, depends, depends.
The middle fed injectors ARE intended to be surrounded by fuel flow and fuel cooled.
Top fed are great for far less external fuel leaking possibilities but will heat soak to malfunction badly on some engines like I-6 Jeeps and 300 CID Fords with the same side under the intake manifold rising exhaust manifold heat.

I would trust the real world expereince of those having ran them dry on the system types they used.
Different systems WILL yield different results.

As the really good auto techs say, “It’s still all just nuts and bolts!”. Means you break it, then you can always just recover down at the take it apart and fix it nut and bolt level and never do that “murder-fi-cation” again!

Steve Unruh

1 Like

Not sure but I think if the fuel pump circuit is disrupted the injectors will not fire. Im pretty sure the ECM or PCM needs to see a signal from the fuel pump in order to fire them. In most cases its been a long time sinse I was a tech so I could be wronge.

hi, all new to the site,

matt i don’t think your far off
if i remember my fuel systems class correctly the pcm has to see fuel pressure or the fuel injector wont fire. let me double check my text book.

just looked, Tim is right on the money

most fuel injectors should be piezo electric kinda like a BBQ grill ignitor except in reverse, hydraulic pressure in the fuel rail is used to open the injector nozzle when the solenoid is switched. the injector is held closed by spring pressure. so even if the pcm tells the injector to fire i couldn’t anyway the piezo actuator just controls fuel inlet, the fuel pressure is what actually opens the injector. hope that helps

Just don’t use a plastic intake manifold. TBI should have wood gas under and not through the injectors as they rot. TBI also has to be turned of along with the fuel pump or you will pull gasoline through at times. We have about 20k miles on multi port fuel injectors with no problems yet. Just a few days ago I ran the Ranger 4 cyl. down the highway with two guys and the AC on full blast doing 78 mph on wood gas with a Keith gasifier. What a blast. had to back off due to traffic.

I’m considering a long bed extended cab 3/4 ton 1995 GMC with a 5.7 TBI engine.

I see Bruce’s warning about sending wood gas through a TBI. I have also read of the sometimes arduous task of fabricating an airtight mixing box to mount underneath the factory TBI. A third opinion I have heard is to just send the wood gas through the TBI, and keep a couple of spare TBIs on hand to swap out - treat it like routine maintenance - whenever you change the oil, change the TBI.

I have limited fabrication skills, and don’t have a professional grade shop or tools - just basic stuff. I am leaning more toward the third option since it seems more doable for us shade tree weekend mechanics.

Has this option worked for anybody?


Yes, check out Dustin Moore’s truck. Also I believe Henry Austin’s truck is very similar.

Hey Chris you beat me to it… heres the link http://driveonwood.com/blogs/dustin-moore/1990-gmc-serria-43l-tbi-now-57l-350

this is the most relevant on TBI configuration http://driveonwood.com/comment/5134#comment-5134

I’ve been running a 5.7 TBI from a 94 3/4ton in my 1500 sierra on woodgas. Have roughly 3500km on woodgas and still going strong. I have the woodgas going through the TBI, I found that trying to plumb it under the TBI was a real chore, it has to be very very tight. See what happens to your idle when you pull off a small vacuum line… now think about putting a large box between the TBI and intake manifold without any vacuum leaks… I found this very difficult to do.

Seems better to suck it up and be satisfied that eventually you will have to clean the TBI (I’m thinking probably every 5k). Need to clean the intake anyway at some point so might as well make a day of it.

I have cleaned the TBI after an engine swap and it took very little time maybe 30min. There isn’t alot to the TBI, 3 bolt and vacuum lines so once you have put it on several times it become very quick. probably best to have a spare on hand so you can clean when it is convenient, nice thing is TBI from 4.3, 5.0, and 5.7 from 1990 - 1995 are all the same except for the injectors. Must have injectors from 5.7 or else you will be running lean from 5.0 or 4.3 injectors… but still running. I ran the 5.7 with 4.3 injectors for a bit while I sourced a 5.7 computer and tbi so it will still sun

another note on your potential build alex. That 3/4 ton seems like it may be very heavy. I am running 5.7 TBI in a 1500 1/2ton 2wd with 5sp trans gmc serria. It’s got good low gearing but I can’t use the overdrive unless I’m running hybrid (i.e injectors on and fuel pump off). I can cruise at 90km/hr on the flat 100% woodgas but 100km/hr for long periods of time is asking a lot of the gasifer… The real limiting factor in the equation is the gasifier I built (18" firetube) at 100km/hr the motor pulls hard on the gasifer and things start to heat up. Not sure what the breaking point is but I know that the heavier the truck the more gas you will need (i.e the harder you have to pull). if you are building the 24" tube then what I have been told you will be ok.

I like the TBI and have been looking to change my stock cylinder heads to vortec 062 ones from a 99 GMC pickup apparently they flow better and will also increase compression. plan to use an aftermarket intake and 454 tbi. Should up my woodgas intake but still won’t help my undersized gasifer problem… will have to build another.