Author Topic: Efi that acts like a carb  (Read 2966 times)

Offline Spoonbill

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Efi that acts like a carb
« on: October 21, 2017, 11:42:46 PM »
Hey all, been a while. 87 r10 305 efi 700r4. We have had some discussions about the efi and how it will cut out and sputter just like a carb does when it loads up. Well now that it is getting colder it is only getting worse. It started getting colder last night and been cold all day. This afternoon I went to run an errand and when I started it up it was just exactly like a carb in the winter time with a carb. It was cutting out so bad that if I put it in gear it would just die so I left it running and let it warm up and then it was ok, not really ok but running like it has been so at least I could go. I have looked at everything on the net and talked to everyone I know to see if anyone might have a clue on what I need to know to fix this and I came up with blanks so now I am asking you all to see if anyone has the slightest clue what might be wrong. Any help would be great guys. Thanks

Offline VileZambonie

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Re: Efi that acts like a carb
« Reply #1 on: October 22, 2017, 08:07:07 AM »
Probably the worst thing you can do is ask everyone on the web  ;D
Facebook is the worst, I see so many bad recommendations just randomly cast out there. If you were to follow this advice it would be much cheaper to find a qualified GM Tech to fix it for you. You should always follow strategy based diagnostics:

Do your preliminary checks, vacuum leaks including at the throttle body and map sensor vacuum hose, ignition system integrity, fuel pressure, isolate the EGR system by disconnecting the hose and plugging it at the EGR valve. Make sure fluid levels are correct including coolant level as the ECM looks at ECT as a primary input.

Check for DTC's, if someone was in there unplugging this and that, clear the DTC's and recheck. Do you have a scan tool? You need to look at data stream, if it does it cold and hot check ECT input, MAP voltage. TBI systems are very simple but you need to have an understanding of the fundamental principal operation. See below:

The Throttle Body Fuel Injection System provides a means of fuel distribution for controlling exhaust emissions within the required limits by precisely controlling the air/fuel ratio under all operating conditions. This is accomplished by means of an Electronic Control Module (ECM) which receives electrical signals from various sensors indicating engine operating conditions, and varies the fuel delivery time (pulse width) of the injector(s) accordingly. The ECM may modify the fuel pulse to compensate for special operating conditions such as cranking, cold starting, altitude acceleration and deceleration. By increasing the injector pulse, more fuel is delivered and the air/fuel mixture is enriched. When the injector pulse is decreased, the air/fuel ratio is decreased.

The ECM is capable of diagnosing faults with the various inputs and systems it controls. When the ECM detects a malfunction, a "Check Engine" or "Service Engine Soon" lamp in the instrument panel is illuminated, warning the driver to have the system checked.

On fuel injected V6 and V8 applications, the throttle body injector (TBI) is located on the intake manifold where fuel and air are distributed through a double bore throttle body. Combustion air is controlled by dual throttle valves while fuel is supplied by dual TBI unit mounted injectors. Both injectors are controlled by one ECM.

When the ignition key is turned on, the ECM will energize the fuel pump relay. The fuel tank mounted fuel pump pressurizes the system to approximately 10 psi. If a distributor reference pulse is not received by the ECM within two seconds, the fuel pump relay will be de-energized, turning off the fuel pump. If a distributor reference pulse is later received by the ECM, the fuel pump relay will be re-energized and the pump will resume operation.

If a fuel pump relay failure should occur, the oil pressure switch can also operate the fuel pump after the engine has built up approximately 4 psi of oil pressure.
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74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline Stewart G Griffin

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Re: Efi that acts like a carb
« Reply #2 on: October 22, 2017, 08:12:42 AM »
i can't help, but i will add that my cavalier, which is TBI, also did this when the engine was cold (and it was cold weather) and would even stall out.  But once the engine warmed up there was no problem.

So maybe it's a TBI thing?

Offline VileZambonie

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Re: Efi that acts like a carb
« Reply #3 on: October 22, 2017, 08:34:36 AM »
If you are suggesting it is a normal "thing" then no.
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74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline Stewart G Griffin

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Re: Efi that acts like a carb
« Reply #4 on: October 22, 2017, 09:11:24 AM »
No, i meant "thing" as in bad thing or glitch.

Now, it use to do it, but then it cleared up, so i don't know what fixed it?

Offline Spoonbill

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Re: Efi that acts like a carb
« Reply #5 on: October 23, 2017, 12:43:40 AM »
I don't have a scan tool but I did have it scanned and no faults. Put in a new iac about 6 months ago cause I thought that might be the problem. I just don't know anything about efi so I feel lost all the time. The hard part for me to wrap my head around is that it only has 67,000 original miles so it just makes you think that there shouldn't have anything wrong. Here is something funny about it. I was thinking maybe it wasn't getting enough air or fuel so I started with checking air. I turned the air cleaner lid upside down and tightened the wing nut which would basically turn it into an open element air cleaner and what do you know, it took the cutting out and cut it in half so now it makes me wonder if the other half is fuel related. It still doesn't make any sense why there would be something wrong with that few miles on it

Offline Spoonbill

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Re: Efi that acts like a carb
« Reply #6 on: October 23, 2017, 12:54:23 AM »
I forgot to say this but the oil pressure gauge just stopped working. It was just fine and when I went out to start the truck so it could warm up so I could go somewhere the needle was buried all the way past the top psi number. Don't know if it is just happening at the same time as the other stuff or if the cutting out is making it do it. I don't know if anyway to check it so I guess I will just have to buy another one.

Offline VileZambonie

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Re: Efi that acts like a carb
« Reply #7 on: October 23, 2017, 06:16:17 PM »
Check the connection at the sending unit.

Remove the air cleaner altogether and plug the vacuum hoses. Check around for vacuum leaks. Spray some carb cleaner at the base of the TBI and see if it changes engine speed with it running.
,                           ___ 
                         /  _ _ _\_
              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)

74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10