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Old 09-15-2004, 09:19 AM   #1
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Nasty part throtte stumble (91 L98)

I've got a nasty part throttle stumble/hesitation on my 91 Corvette. I assumed it was a tps, so I yanked the TPS off my IROC last night.

It still has the problem, although it does appear to be less severe. Now I'm wondering where to look next. My gas milage is still good (20mpg), so I don't think that I'm having an oxygen sensor issue. Everything looks good on my scan tool. O2 crosscounts are up, engine temp indicates that the temp sensor is functioning properly. How do I determine whether the MAP sensor is working right?

Whatever it is seems to only effect part throttle. WOT (open loop) is fine, and light throttle & crusing is fine.

Although this TPS seems to exibit less of a stumble than the old one, the IROC has never had a stumble, so I have to assume that the TPS is ok.

Thanks.
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Old 09-15-2004, 10:50 AM   #2
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Vacuum leak? How is fuel pressure, does the vacuum bring it down at idle?
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Old 09-15-2004, 12:13 PM   #3
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I haven't checked vacuum, but it doesn't exibit any of the typical symptoms. The car idles beautifully.

Wouldn't a fuel pressure problem show up at WOT too?
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Old 09-15-2004, 12:33 PM   #4
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Probably, just trying to see what is up. I would have gone for the TPS like you, since that not it, it is time to go fishing! How about timing, if to far advanced, it would idle but start to miss once the ECM started adjusting at higher RPM. Just tossing out ideas.
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Old 09-15-2004, 01:18 PM   #5
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unless the timing changed on its own, it shouldn't be that. I've never touched the timing on this car. Plus, it doesn't do it at WOT. Timing would effect WOT.

Thanks for the suggestions though. If nothing else, it's forcing me to rule out a bunch of possibilities.
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Old 09-15-2004, 02:38 PM   #6
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This is from the Holley 950 Commander manual. It should work for any 1 bar MAP sensor system though. I hope this helps.

Testing the Manifold Absolute Pressure (MAP) Sensor (1 Bar)
One way to check the MAP sensor is to monitor the system with the software. Check the map sensor readings with the engine
turned off. It should be fairly close to 101 kPa at sea level and decrease by about 3.4-3.5 kPa per 1000 feet elevation above
sea level. The MAP value may also vary a small amount due to weather conditions. The table below shows MAP values in
kPa compared to the more familiar units of in Hg. The second column is gauge pressure in Hg and the third column is
vacuum in units of in Hg.
MAP Value to Manifold Pressure Value to Vacuum Comparison
MAP Value (as read on the
computer)
Manifold Pressure Value (in Hg) Vacuum (in Hg) Relative to Ambient
Pressure Condition (sea level)
101 kPa 29.7 0.0
90 kPa 26.5 3.2
80 kPa 23.5 6.2
70 kPa 20.6 9.1
60 kPa 17.6 12.1
50 kPa 14.7 15.0
40 kPa 11.8 17.9
30 kPa 8.8 20.9
20 kPa 5.9 23.8
NOTE: Users at higher altitudes will have MAP values lower than 101 kPa as described in the above paragraph along with a
lower corresponding manifold pressure. However, the vacuum on a typical vacuum gauge is referenced to ambient
conditions and will read 0.0 with no engine vacuum regardless of MAP value unlike the sea level based third column
in the above table. A MAP value lower than 20 kPa is most likely an indication of a bad MAP sensor connection or a
MAP sensor problem.
1 - If you do not have a PC, use a digital voltmeter and tap into the ORANGE wire at the MAP sensor. With the engine off
and the key in the run position, voltage should be close to 2.0 volts.
2 - Connect a hand vacuum pump to the MAP sensor with the key in the run position. Using the vacuum pump, the MAP
sensor can be checked at the following vacuums. Voltage output should be close to these values. The following values
are for a 1 Bar MAP sensor.
20 Hg vacuum 1.1 volt
15 Hg vacuum 2.0 volt
10 Hg vacuum 2.8 volts
5 Hg vacuum 3.8 volts
0 Hg vacuum 4.6 volts at sea level (less at altitude)
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Old 09-15-2004, 03:09 PM   #7
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Just off idle?

More pronounced when cold or the same when warmed up fully as well?


IF the TPS didn't change it....

I'll take..

"Crappy hard plastic vacuum lines leaking somewhere in the engine compartment" for $500 Alex...
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Old 09-16-2004, 10:41 AM   #8
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It's not "just" off idle. I can give it enough throttle to accelerate at a modest pace. It's if I try to go a little more that it starts.

The last few days (since changing the TPS), it has been considerably less severe, which is pissing me off. I expected the TPS to either fix it, or don't fix it. This inbetween crap doesn't allow me to rule it in or out.
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Old 09-16-2004, 02:35 PM   #9
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You did say you took the TPS off your IROC right? Maybe try buying a new one to see if it fixes the rest of the problem. The Iroc TPS could have dirty contacts as well, just not as bad.
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Old 09-16-2004, 02:35 PM
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