tachometer reading 98 gts?? fuel pump??

[subgunman]

Hello,
Keith from all pro just called me and informed me my tachometer is reading incorrectly, but the engine is idling where it is supposed to. It is idling at 672 rpm per his computer, but the tach shows it just below the SECOND hash mark between 0 and 1.
The first hash mark is very close to zero, and the second is halfway between 0 and one. can anyone tell me if that is indeed the 500rpm mark???? I find it odd that there is a mark so close to zero, then only one more between 0 and 1.
Also he informed me my fuel pump is bad, sucking air with fuel into the motor. Has anyone heard of this causing an excessive engine shimmy?
thanks in advance!

 

[99GTSACR]

1st mark past 0 is 500 RPM. 2nd mark is 750 RPM. Sounds like his tach is normal.

 

[Janni]

OK - that sounds weird. How does your fuel pump **** air from a tank full of fluid?

Are you sure this person is familiar with how the lopey cam'd 1996-1998's idle - my engine shook like a son of a b and acted like it was going to stall. I loved it - and it was running PERFECTLY.

 

[CROM]

This is my car idling. Completely normal. Notice the door shaking.

 

[99GTSACR]

OK - that sounds weird. How does your fuel pump **** air from a tank full of fluid?

The only way air could be sucked in is if the fuel level was below the fuel pump "and" the pickup line had a leak.

Does the car run different if the fuel tank is full vs near empty?

 

[subgunman]

UM.......NO I suppose not. I was told that at 50lb it causes a "cavitation" and allows some air......
This is the guy on a previous post here a few days agothat so many recomended, keith from all pro automotive here in dallas. Now I am wondering.......

 

[ewave]

Cavitation is a term that is usually used to describe what happens to the leading surface of an impeller when it is rotated at excessive speed in a liquid. A common place for this to occur is on a boat's impeller. What happens is that the fluid can not accelerate fast enough to follow the leading surface of the impeller, and it separates from the surface- causing a vacuum. The water then collapse on the vacuum, and crashes back into the surface of the impeller. This creates a lot of noise, and is very destructive. It also prevents the impeller from operating efficiently.

I suppose it would be possible for a fuel pump impeller to spin too fast. If this were the case, sufficient fuel pressure would not be reached at the outlet of the fuel pump. "Air", (Oxygen, Nitrogen, and Co2) would not be generated from the cavitation.

I own an 97 GTS with a very lopy idle. I live in far North Dallas. If you would like to meet, I would be happy to compare car and idles with you. I also have a ODBII diagnostic tool that I have used to diagnose problems like this.