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Fuel Issue: Can't tell if it's loading up or starving

Old 01-08-2019, 11:57 AM
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Fuel Issue: Can't tell if it's loading up or starving

***SOLVED***: Recalibrated the TPS, replaced the coils, wires and plugs and issue went away. ***

During my last two race weekends I've experienced some sort of phantom fuel issue. The first was at Roebling Road back in October. The field was formed up and we were proceeding down the front straight, I was around 5000 RPM in 3rd, green flag dropped, I planted it and "Bop Bop Bop Bop Bop". It sounded and felt as if I was hitting the Rev Limiter. The car lagged and front end dropped as if I had it hit a wall, or the Rev Limiter. But it the tach was still around 5K. I quickly shifted up a gear and the problem went away. I didn't experience the problem for the rest of the weekend.

This weekend at Homestead-Miami Speedway the problem returned. Though not on the start or straights. This time on the exit of Turn 2/entry to Turn 3 and the exit of the Hairpin. Didn't matter which gear I was in or where I was at on the RPM band. It was whenever I would try and go back to power. It felt like the car had loaded up on fuel. I go to the back straight and blipped it high a couple of times, but didn't seem to clear anything or solve the problem for the rest of the weeknd. Also the problem didn't cure itself by simply backing off and going back to throttle. In Turns 2,3 & 4 the problem persisted as I tried 2nd, 3rd and 4th gear. The RPMs didn't skyrocket, so I don't think it's clutch slippage. The issue also wasn't consistent, it didn't happen every lap.

The tank was also full when I went out, so I don't think it's from fuel slosh/starvation.

The Prep shop is checking all the fuel filters and pumps.

Anyone else experience anything like this, have any insight, or think they might now what the problem is?

Last edited by TORacing; 01-14-2019 at 01:59 PM.
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Old 01-09-2019, 08:09 AM
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I can't prove this, but I have had the same experience a few times. Here are what I believe the be the causes:
  1. Fuel starvation. At below 1/3 tank on the stock fuel system, you will run into starvation on long right sweepers.
  2. Limp mode. When I have overshot the rev limiter more than once in the same drive cycle (usually trying to stay on the gas during corner exit and waiting to balance the load before shifting), the ECU goes into a pseudo limp mode and bogs down as I reach something around 5500RPM. Disconnecting the battery clears it. It is not the same as full limp mode, and I can't find any documentation on such a thing existing, but it has happened to me twice now.
Any chance you rode the rev limiter too hard more than once in the same drive cycle?
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Old 01-09-2019, 10:37 AM
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Steve, we're running a full Motec system and standalone ECU. So it shouldn't be limp mode. Also the fuel tank was full when we went out. My first thought was starvation though from the fuel sloshing to one side of the tank. But it also happened on a straight away well after the turn. I'm starting to think it may be the TPS. It seems to happen after going from power, easing off and back to power. If the TPS is sending bad signals it would cause the acceleration to hesitate, I don't know if it would cause the brap brap brap that comes with the Rev Limiter? But I'm guessing it could.
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Old 01-10-2019, 06:46 AM
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can it be ignition issues? can you log TPS and Actual Throttle.
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Old 01-10-2019, 07:24 PM
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Are you using the stock throttle body?

Motec driving it?

What have you done for fuel system?

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Old 01-10-2019, 07:46 PM
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Originally Posted by trackjunkie View Post
can it be ignition issues? can you log TPS and Actual Throttle.

We ordered a new TPS, so we're gonna switch that out and see if it solves the problem. We put it on the dyno yesterday and weren't able to replicate the issue. We can't pull up old data from the on-track session off the MoTec. The MoTec is from SpeedSource's GrandAm days, and MoTec doesn't even have the software to access the data logging anymore.
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Old 01-10-2019, 07:48 PM
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Originally Posted by dannobre View Post
Are you using the stock throttle body?

Motec driving it?

What have you done for fuel system?
Yes stock throttle body, with a 55mm restrictor plate for SCCA STL. Yeah the Motec is driving it. The fuel system is stock, except for the modifications that SpeedSource made to the tank for the GrandAm series. Still uses factory pumps and what not.
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Old 01-12-2019, 07:22 AM
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Every time I've had a fuel starvation issue the car just died. Ignition issues seem more likely to cause stuttering an popping.
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Old 01-14-2019, 01:58 PM
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We recalibrated the TPS, replaced the coils, wires and plugs and issue went away
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Old 01-14-2019, 02:02 PM
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that's good. should have done those separately too narrow down the cause in case it happens again.
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