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Thread: Acceleration is sluggish for a GMC Motorhome.

  1. #11
    Join Date
    Dec 2009
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    Connecticut
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    Quote Originally Posted by mrtoadsc View Post
    How do I know when learning is complete?
    Basically, when the Learn Table no longer makes significant % changes.
    This forum thread answers more Learn Table related questions:
    http://forums.holley.com/showthread....el-Learn-Table (Learn Table Discussion)

    On a related note: How do I adjust the "Target A/F Ratio Table" to make it richer under "Load". The online docs seem to be out of date for this screen.
    I'm not sure I understand. You just change the AFR numbers in the table cells desired, and click "Save".

    From what I can determine the AFR may be too lean. My mechanic is convinced this is a fuel starvation problem, so we are going to investigate that.
    The fuel pressure has to be right, period. Do you have the Holley fuel pressure transducer installed?
    The fuel & oil pressure transducers are completely optional. The engine runs the same with or without them.
    The fuel & oil pressure transducers are used for Data Monitoring, Data Logging and setup Outputs based on fuel & oil pressure.

    This is not to be confused with the "Actual System Pressure" fuel parameter (in Engine Parameters),
    which must be entered accurately because the ECU does use this single value for lb/hr calculations.
    With TBI, the Actual System Pressure is measured anytime engine is running (no vacuum reference hose), or key-on/engine-off pump running.
    With MPFI, the Actual System Pressure is measured with the vacuum reference hose temporarily disconnected, or key-on/engine-off pump running.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  2. #12

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    Quote Originally Posted by 440roadrunner View Post
    Something bothers me about this entire thread...you said "high performance." That is a very "loaded" statement. Is it possible this is the entirely wrong engine for this application? Too high compression ratio? Too large camshaft? Just how big and heavy is this motorhome?
    My only statement about "high performance" is that this is a non-stock Oldsmobile 455 engine. The compression is 185 psi while the stock was 150. This isn't a huge motorhome: 23 foot GMC, which weighs about 10,000-11,000 lbs when loaded.

    The engine ran with power (and issues) when we had the other carburetors: Rochester Q-jet and a modified Holley 4150. I like the driveability and easy starts with the EFI. The challenge is to get the power back. There may be something fundamental that we are missing here.

  3. #13
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    Quote Originally Posted by mrtoadsc View Post
    The compression is 185 psi while the stock was 150.
    He said "compression ratio", not cranking compression.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  4. #14

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    So I'll chime in a bit. The problem we are having is too lean at high load/low rpm. It doesn't pull like the carb did; instead it stumbles & pops through the throttle body. Lean condition. The AFR Learn did confirm this for us on our graph. We increased the map and solved quite a bit of the problem, but still see a stumble at about 40% throttle, and about 1500-1800 RPM, like if your cruising down the street at 30-35 mph and trying to accelerate.
    I saw one of the TPS charts starts off adding fuel till 60 or 65% of the throttle, then is below 100%, so it would then lean it out.

    If it feels just as if an accelerator pump is not working at 1/3-1/2 throttle what would you suggest to increase? TPS Rate of Change - right? Or be sure the 'TPS curve or cam' is still present at that throttle?

    There is quite a bit of a learning curve for me. I'm more used to short and long term trims and OBD2 SAE, info like O2 sensor voltage, MAF sensors and such.

    And I'm starting to think if it's rich enough to drive OK, then the Learning will fine tune from there over time right?

  5. #15
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    Quote Originally Posted by Smittylube View Post
    If it feels just as if an accelerator pump is not working at 1/3-1/2 throttle what would you suggest to increase? TPS Rate of Change - right?
    Yes, but Acceleration Enrichment can't be tuned until the Base Fuel Table is steady state tuned.
    http://forums.holley.com/showthread....9444#post29444 (Additional Information)

    And I'm starting to think if it's rich enough to drive OK, then the Learning will fine tune from there over time right?
    Yes. However, the Learn Table self-tunes in accordance to the Target A/F Ratio Table. So that must be correctly programmed first.
    http://forums.holley.com/showthread....0975#post50975 (Additional Information)
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  6. #16

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    OK, I'll go that direction. Makes sense.
    With that being said, are the default enrichment curves close to what you end up with, after proper tweaking? (Typically if there is a typical situation.) Thanks, Steve

  7. #17
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    Quote Originally Posted by Smittylube View Post
    With that being said, are the default enrichment curves close to what you end up with, after proper tweaking?
    I'm multi-port fuel injected now, and I don't remember what the AE tables were like when it was TBI.
    I can tell you when the Target A/F Ratio Table/Base Fuel Table is well tuned, not a lot of AE is required.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  8. #18

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    OK I get it. This is a large displacement engine, but with the auto transmission, it can keep the RPMs low. Also, due to the size of it being a motorhome at the low rpm, it's a full load condition and needs plenty of fuel to start to spool up. Thanks for your help. Steve

  9. #19
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    Reading this thread got me thinking of a similar problem I had. However, I want to understand what your issue is before I mention the wrong/right thing I did. I'm guessing you have an acceleration problem, but the issue is really at sustained throttle position, or RPM range just after acceleration? In other words, you can accelerate OK, but trying to sustain the range is where your issue is. Correct or not? You can accelerate fine, but then trying to sustain that RPM range makes the engine go rich/lean & pop/fart all over unless you take some foot out of it?

  10. #20

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    Danny: Thank you for your help and for providing guidance to "Smittylube", who is my mechanic on this project. I'm just about to head out on a two week trip and we are planning to revisit this issue in September.

    In the meantime, Smittylube has downloaded the ECU client software, and I provided him data & configuration, and he will be getting familiar with the system.

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