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Throttle Body injection operation?

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Post Options Post Options   Thanks (0) Thanks(0)   Quote jkilbert Quote  Post ReplyReply Direct Link To This Post Topic: Throttle Body injection operation?
    Posted: October-06-2013 at 11:06am
On a TBI set up the injectors flow continuously just as a carb flows fuel continuously. On port type systems tthere is a cam or crank sensor that sends info to the ECU to fire the appropriate injector at the right time along with controlling spark. Most TBI systems run 45lbs of fuel pressure for the injectors to open properly. In a TBI the ECU adjusts the fuel flow whereas in a carb the amount of air passing through the venturi regulates the amount of fuel. So a couple routine things to do first, you said you had fuel so check for adequate pressure,also check for spark. Also check all fuses, relays and connections. Also on fuel injection systems there should be a return line.
I know this is overly simplified but all TBI is is a high tech electronic carb. Aside from the common checks to make one failure point in the system that will cause it to not run is the throttle position sensor. The O2 sensor with cause it to run poorly but still run, the same with the temp and knock sensors.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote phatsat67 Quote  Post ReplyReply Direct Link To This Post Posted: October-02-2013 at 4:38pm
True, we're just lucky they were scabbing new tech onto old engines so it isn't hard to revert back to the tried and true methods.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dreaming Quote  Post ReplyReply Direct Link To This Post Posted: October-02-2013 at 3:48pm
Good info!   thanks Zach I'll bend up a fuel line for it and give the carb a shot.   I too would see the cool factor of keeping the injection, but don't want to spend extra money to find out that the ECU is the problem, then be down the carburetor road anyway.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote phatsat67 Quote  Post ReplyReply Direct Link To This Post Posted: October-02-2013 at 1:56pm
That's the issue of the Protec and pro boss engines. They rely on the Protec to function properly. Sounds like the ignition works right. My understanding is that the "puck" where the distributor should be acts as a Camshaft position sensor for the computer to know when to fire injectors. If the ignition fires properly then I would bet you are looking at a computer or injector problem. Unlikely that all of the injectors are bad at once. It would be neat to get it running right but from a cost standpoint and longevity a carb probably looks better in the long run.

The pump should work fine for a carb motor. The later HO 454 ran this pump. The ho 454s require a different fuel line because of this. Found out when I was snooping all of the other fuel lines PCM offers.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dreaming Quote  Post ReplyReply Direct Link To This Post Posted: October-02-2013 at 1:27pm
I do have a local dealer, If I decide to pursue FI at a later date, I will have it checked out.   My current goal is to make it run so I can determine whether the block is cracked or not, so I want to get it running with as little expense as possible.   As of last night, I have the TBI removed and a carb installed, but have yet to run a inlet line from the pump.    I think the carter high volume pump is going to be OK to supply the carburetor(4-7PSI) but I haven't checked it yet. I need to put a gauge on it before connecting to the carb.   I'll probably order a mechanical pump in the next week or so to give it a complete factory carburetor set up.   With cold weather knocking on my door, I need to winterize soon, so I need to get all my troubleshooting done ASAP.    
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Double D Quote  Post ReplyReply Direct Link To This Post Posted: October-02-2013 at 12:55am
During cranking, I'm not 100% sure. You would think they should be, and yes, they should be squirting gas when cranking.

I don't know of a way to isolate and test. I have two injectors and one was sputtering when it got hot which would make my engine surge during idle. I was able to switch wiring and the problem followed the wiring so it wasn't the actual injector. So the dealer confirmed the wires were good (I assume an ohm test) which left only the computer.

Do you have a local dealer who can test?   Computers cost plus $750.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dreaming Quote  Post ReplyReply Direct Link To This Post Posted: October-01-2013 at 11:47pm
I don't think the injectors are clicking at all.     Should they be clicking during cranking?   I assume that they should show some indication of spraying fuel during cranking?, but there is no indication of that currently. Can I isolate injectors vs. wiring/computer issues by manually firing the injectors?   if so, how would I do that? voltage?

My boat is a 1994, with the Pro-boss set up a(high output 351w with gt40 heads, Protec ign & TBI) .   This was factory for my boat/engine combination and from what I understand was phased out in 1995 as it was problematic.    The Holley set up is identical in looks to mine, except for the ECU.   
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Double D Quote  Post ReplyReply Direct Link To This Post Posted: October-01-2013 at 11:23pm
I really don't know anything about a return line, and I have a different manufactured boat, but if you know your getting fuel too the injectors and then nothing is squirting from the injectors, that sounds like your problem. The question now is what is causing it.   

When you do have it running are the injectors clicking??   If so, they could be clogged.   If not, there not engaging and that could be bad injectors, bad wiring, or a bad computer.

If your system looks similar to the attachment was it an aftermarket add on?   What year boat and engine?

I would like to get another opinion from the peeps on this board with a similar setup. I haven't stayed at a Holiday Inn Express in a while. :)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dreaming Quote  Post ReplyReply Direct Link To This Post Posted: October-01-2013 at 8:49pm
DD,   Thanks for the response

on my boat, there are two fuel lines at the top of the tank, one for supply (attached to the dip tube, anti siphon valve) the other is what I assumed to be a return line.   I could tell there was fuel flow from the pump to the inlet of the TBI unit, as I was getting fuel out of the schraeder valve when I checked the pressure. when I pushed the center of the Schrader, fuel would come out, turn the key, the pumps would run and pressure would resume.

your assumption of my symptoms is correct... I am not getting anything out of the injectors. since everything on this engine is an unknown to me at this point, I am going to put on the carburetor I have and try that way.   I am not sure that going any further with this TBI system is going to be advantageous to me.   

I did find this in my search...   
Holley made a "Pro-Jection" tbi system that resembles the system on my boat.    Here is a link to the manual

Holley TBI manual
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Double D Quote  Post ReplyReply Direct Link To This Post Posted: October-01-2013 at 6:34pm
Originally posted by Dreaming Dreaming wrote:


Can someone help to expand my understanding of the Throttle body System?

- I have fuel flow and pressure at the inlet of the TBI, but no fuel returns to the tank.

- The Anti siphon valve and pickup have been cleaned, tank refilled with fresh fuel (1.5 gals)

- engine starts easily when fuel is poured into the TBI/intake.

- once the poured fuel is gone, The engine dies, and won't restart until more fuel is added.   

I don't know what the TBI is supposed to do, and there doesn't appear to be an explanation or trouble shooting information in the manuals (either the 93 carb manual or the GT40 EFI manual)



You have me scratching my head here... I am no expert on this stuff but one thing is confusing me with what you say: "I have fuel flow and pressure at the inlet of the TBI, but no fuel returns to the tank." Fuel returns to tank??

Here is my understanding of Throttle Body Fuel Injection: You have a somewhat standard carburetor just like older engines but sitting on top of that carburetor are injectors that squirt gas into the top of the open barrels. Those injectors are controlled by a computer. If your not getting fuel out of these injectors a few things could be wrong:
1. Dirty or clogged Injectors
2. Broken Injectors
3. Fuel not reaching injectors
4. Injectors not plugged in
5. Computer problem

If I am off base or not understanding something correctly, I apologize. I did go through an Injector/Computer problem a few years ago.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dreaming Quote  Post ReplyReply Direct Link To This Post Posted: September-30-2013 at 1:34am
one more question while we are on the subject...   Can I use the carter electric (low pressure) fuel pump to run a carburetor? or do I need to get the mechanical fuel pump?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dreaming Quote  Post ReplyReply Direct Link To This Post Posted: September-29-2013 at 11:21pm

Can someone help to expand my understanding of the Throttle body System?

- I have fuel flow and pressure at the inlet of the TBI, but no fuel returns to the tank.

- The Anti siphon valve and pickup have been cleaned, tank refilled with fresh fuel (1.5 gals)

- engine starts easily when fuel is poured into the TBI/intake.

- once the poured fuel is gone, The engine dies, and won't restart until more fuel is added.   

I don't know what the TBI is supposed to do, and there doesn't appear to be an explanation or trouble shooting information in the manuals (either the 93 carb manual or the GT40 EFI manual)

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