Car leans out at 4700 rpms, what could it be?
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Joined: Nov 2000
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From: Newark, DE
Car: 86' Z28
Engine: 355
Transmission: T-56
Car leans out at 4700 rpms, what could it be?
on the way home to day i got on it pretty hard and it started bogging like crazy right around 4700 rpms, so i look at the A/F guage and it's all the way into the lean zone. This has happened to me before and i always thought it was a weak ignition, but wouldn't weak spark cause a rich condition up top? what could be the cause of this?
Eric
Eric
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Joined: Jan 2000
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Zup, Man?
That RPM is about the point where the stock fuel delivery system is about maxxed out. The pump and filter should be checked to make sure, but the injectors themselves are likely at their maximum (80% duty cycle) flow rates at that point.
That's just another piece of granite in the "stone wall" that all TPI systems hit at that RPM range:
1. The ignition spark duration and energy becomes suspect at those RPMs, but missing should cause a rich condition as you surmise.
2. The valves start to float at that RPM under stock springs, even with the "LameAss" factory cam profiles.
3. The induction system itself is nearly out of tune at those RPMs, so overall the TPI system really starts to suffer at anything between 4,500-5,000 RPM.
On the bright side, it looks as if the GM Powertrain guys matched most of the systems pretty closely, eh?
That RPM is about the point where the stock fuel delivery system is about maxxed out. The pump and filter should be checked to make sure, but the injectors themselves are likely at their maximum (80% duty cycle) flow rates at that point.
That's just another piece of granite in the "stone wall" that all TPI systems hit at that RPM range:
1. The ignition spark duration and energy becomes suspect at those RPMs, but missing should cause a rich condition as you surmise.
2. The valves start to float at that RPM under stock springs, even with the "LameAss" factory cam profiles.
3. The induction system itself is nearly out of tune at those RPMs, so overall the TPI system really starts to suffer at anything between 4,500-5,000 RPM.
On the bright side, it looks as if the GM Powertrain guys matched most of the systems pretty closely, eh?
Thread Starter
Supreme Member
Joined: Nov 2000
Posts: 1,760
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From: Newark, DE
Car: 86' Z28
Engine: 355
Transmission: T-56
thanks for the info vader. Although i don't have TPI, you did hit on the two things that i thought were suspect to my problems, which should be the same with a carb.
1. my ignition, i really think that there is a problem there, and being that is all stock and old, im gonna replace everything, and get a module and coil for it.
2.) Fuel system, this was my second guess, so im gonna go with a holley mech. fuel pump and pressure regulator. I'm thinking that these two components are gonna balance themselves out quite nicely and hopefully this problem will be gone.
just to complicate my sitiation even more, when nailing it from a dead stop, it doesn't lean out at all, all the way to the top of 4th gear. However when go to WOT froma roll in third gear it does...
Anybody else have any ideas, or would like to encourage or discourage my choice of parts here?
Eric
1. my ignition, i really think that there is a problem there, and being that is all stock and old, im gonna replace everything, and get a module and coil for it.
2.) Fuel system, this was my second guess, so im gonna go with a holley mech. fuel pump and pressure regulator. I'm thinking that these two components are gonna balance themselves out quite nicely and hopefully this problem will be gone.
just to complicate my sitiation even more, when nailing it from a dead stop, it doesn't lean out at all, all the way to the top of 4th gear. However when go to WOT froma roll in third gear it does...
Anybody else have any ideas, or would like to encourage or discourage my choice of parts here?
Eric
Thread Starter
Supreme Member
Joined: Nov 2000
Posts: 1,760
Likes: 0
From: Newark, DE
Car: 86' Z28
Engine: 355
Transmission: T-56
well i just ordered a new fuel pump and regulator, along with a pertronix flamethrower coil and module. I guess we'll see if that helps any.
Eric
Eric
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Joined: Jan 2001
Posts: 4,415
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From: Surrey, BC
I found I had the same problem with my TBI and all of a sudden my car wouldnt start and I realized it was the fuel pump that caused it. Now my car will rev to 5800rpm with my MSD coil and wires.
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Thread Starter
Supreme Member
Joined: Nov 2000
Posts: 1,760
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From: Newark, DE
Car: 86' Z28
Engine: 355
Transmission: T-56
LOL, coach McGerck kicks a$$.
It seems as though i am hitting the right spot with the new fuel pump, hopefully it will in fact solve the problem...
Eric
It seems as though i am hitting the right spot with the new fuel pump, hopefully it will in fact solve the problem...
Eric
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