GM350 TBI hesitates when accelerating fast...

[fmt2bx]

Gm350 TBI engine. Idles fine, if I apply throttle gently, no issues. However if I try to do a hole shot, motor hesitates, misfires or backfires (can't tell) then accelerates normally.
Also while at 4400 Rpm if I go to neutral rapidly, no misfire or backfire. Works fine.
The only issue is Hole shot.

I did some research on the net but I was wondering if someone could give me a step by step troubleshooting list.

Here is what I have done so far:
- New plugs
- New distributor cap and rotor
- Took the TBI apart, cleaned, replaced with rebuilt kit and replaced the injectors
- Fuel tank is cleaned refilled with 89 octane ethanol free.
- VST removed and cleaned up.

This motor is new to me, I do not have any previous experience with it.


Thank you.

Frederic

 

[Scott Danforth]

what is your fuel pressure?

 

[fmt2bx]

what is your fuel pressure?

Going to check today. Will report results.
thanks

 

[Scott Danforth]

TBI is a 1 bar system, so your fuel pressure should be 15psi at all times

 

[alldodge]

Never seen pressure that low (15 psi) on a Merc. Depending on the serial number it would probably be 30 psi

An issue to measure TBI is getting the fitting to connect inline

 

[Scott Danforth]

Never seen pressure that low (15 psi) on a Merc. Depending on the serial number it would probably be 30 psi

An issue to measure TBI is getting the fitting to connect inline

you are correct, its a 2-bar system. it should be at 30 psi. (was thinking the automotive side at 1 bar)

 

[Lou C]

anti siphon valve checked? this is a frequently overlooked cause of fuel starvation issues.

 

[fmt2bx]

you are correct, its a 2-bar system. it should be at 30 psi. (was thinking the automotive side at 1 bar)

Hi. Thank you for the replies.
Went this morning and got only 6PSI at VST and 11PSI at the fuel line( see pix).
I guess it’s way too low..

 

[fmt2bx]

Definitively not enough pressure, if this is where I am suppose to check it...

I was trying to get some specs on the TBI injectors and the fuel diagram of our host doesn't show a part # for the 2 injectors???

Anybody knows what those injectors should be? I want to verify that I have the right ones. Thank you.

 

[Scott Danforth]

Definitively not enough pressure, if this is where I am suppose to check it...

I was trying to get some specs on the TBI injectors and the fuel diagram of our host doesn't show a part # for the 2 injectors???

Anybody knows what those injectors should be? I want to verify that I have the right ones. Thank you.

View attachment 328180View attachment 328179View attachment 328178

they are GM injectors, and GM only published the whole TBI unit. however there are specialty shops you can send the TBI unit to in order to get it serviced.

 

[alldodge]

The way to find out what your motor uses is with the serial number? We don't even know what year the motor is?

Being a VST TBI, your pressure should be 30 psi and is measured using adapter 91-806901
Your low pressure pump is working seeing 6 psi, the HP pump is not
The VST can be taken apart but be real careful. Should be able to find the seal kit, but the screen on the bottom of the pump can be brittle

 

[fmt2bx]

The way to find out what your motor uses is with the serial number? We don't even know what year the motor is?

Being a VST TBI, your pressure should be 30 psi and is measured using adapter 91-806901View attachment 328200
Your low pressure pump is working seeing 6 psi, the HP pump is not
The VST can be taken apart but be real careful. Should be able to find the seal kit, but the screen on the bottom of the pump can be brittle

Thank you very much.

 

[fmt2bx]

First I would like to thank you all for all your help. This motor was rebuilt in 2003 by Quicksilver and this is the only info I can find on the motor (see pic).
It isn't the ideal situation where I could tell you what year and SN this motor is. Bottom line is, it looks like at some point the motor was rebuilt and all the accessories were put back on the motor.

I have spent a lot of time trying to figure out how this thing works. This is my first I/O motor and it was quite a learning curve.
I am still unclear about how this motor came to be... GM develops the motor, Volvo , Mercruiser etc... Marinize the engine. Does this mean that volvo or mercruiser designed their own ECM? Looks like GM designed the TBI injectors and no one else. Should the fuel pumps outputs differs from manufacturer to manufacturers ?

Here is what I have found in the Volvo manual.. When ignition is turned ON (not cranking) fuel pumps pressurize system for 2 sec. The book says that for 5.7L TBI the pressure should be between 9 and 13 psi. Now where is this pressure read??? On top of VST or in the HP line going to TBI?
I have a schrader valve on top of VST (A) and a schrader valve on the HP line (B). The manual does say where to check the pressure. Now if the pump is rated at 30PSI how come you get only 13psi reading?
Now, also in the manual, it says that inside the TBI throttle body is a pressure regulator (which I have replaced) and that regulator keeps 13psi to the injectors. I don't see any place in there where I could get a reading, unless it is in the return line.

Guys, I am very sorry to be asking all these questions but it doesn't make sense at all to me. I don't mind changing the fuel pumps at all if that's what it takes and I am not second guessing your knowledge, I just would like to understand so I can troubleshoot better.

Thanks again


.

 

[alldodge]

OK you got an old one. Since its a repower then would need to know the year of the boat, because that would help get in the ball park. Being that old its a Gi, GSi or GLi because they are the only ones which list HP at 11 +/- 2 psi (9 to 13)

GM makes the motor and Delphi makes the ECM. VP and Merc program the ECM for there motors. This being a VST TBI I would guess it should have a MEFI-1 ECM.

The HP fuel pump can produce close to 100 psi [EDIT; should be less then 100] but its the regulator that keeps it at a specific level. There is no issue with using a Merc motor and VP ECM, so long as parts have not been swapped or interchanged.

Look for the part number on the injectors and the ECM

 

[alldodge]

Disregard the 100 psi HP pump, its probably less but would be much more then 13

 

[fmt2bx]

OK you got an old one. Since its a repower then would need to know the year of the boat, because that would help get in the ball park. Being that old its a Gi, GSi or GLi because they are the only ones which list HP at 11 +/- 2 psi (9 to 13)

GM makes the motor and Delphi makes the ECM. VP and Merc program the ECM for there motors. This being a VST TBI I would guess it should have a MEFI-1 ECM.

The HP fuel pump can produce close to 100 psi [EDIT; should be less then 100] but its the regulator that keeps it at a specific level. There is no issue with using a Merc motor and VP ECM, so long as parts have not been swapped or interchanged.

Look for the part number on the injectors and the ECM

Ok thanks. I will look for the part #.

I replaced the MAP sensor, no luck. I am going to do another compression check and start over..
Today I kept the fuel pressure gauge connected while drive testing, the 11 PSI is steady, never drops even when engine is acting up. Also, if throttle applied slowly, no issues, motor never misses and fuel pressure still at 11 psi.

Here is a video of the issue:

 

[ChrisCraftFan]

Gm350 TBI engine. Idles fine, if I apply throttle gently, no issues. However if I try to do a hole shot, motor hesitates, misfires or backfires (can't tell) then accelerates normally.
Also while at 4400 Rpm if I go to neutral rapidly, no misfire or backfire. Works fine.
The only issue is Hole shot.

I did some research on the net but I was wondering if someone could give me a step by step troubleshooting list.

Here is what I have done so far:
- New plugs
- New distributor cap and rotor
- Took the TBI apart, cleaned, replaced with rebuilt kit and replaced the injectors
- Fuel tank is cleaned refilled with 89 octane ethanol free.
- VST removed and cleaned up.

This motor is new to me, I do not have any previous experience with it.


Thank you.

Frederic

What Injectors did you use? I am really struggling with location any sort of cross reference.

 

[fmt2bx]

What Injectors did you use? I am really struggling with location any sort of cross reference.

ChrisCraftFan,
Can’t remember but i ll look it up tomorrow and will post back here.

 

[fmt2bx]

Actually here it is:

https://www.amazon.com/dp/B07TC43V4T/ref=cm_sw_r_sms_api_i_NzkOFb2702Y6R

 

[alldodge]

Beware of buying from Amazon