Ok, after installing a fuel pressure gage on my 90 L98 the fuel pressure was set at the AFPR at 51psi. With my small bolt ons ( SLP runners, flowmaster exhaust, vortex ram air, TB coolant bypass, 8 degress timing) I felt that this was too high and set it back down to 46psi after getting recommendations from this forum. It snowed this weekend and have not driven it yet. That said, I noticed that on TPIS website they call out for 49-51 PSI working well on a stock or mild 88-90. Here is the story and then the final question:

I bought the car used 4 years ago and it had the AFPR already on it so never knew what it was set at until now. The car runs well most of the time. Feels GREAT when it is almost warm during the first 10 minutes of driving, really barks out the exhaust and feels strong throught he rpm range. After driving for 20 minutes or more the car doesnt ahve the same feel ( not as strong ) and vauge up top in the rpm range, doesnt even sound as good out the exhaust if that makes any sense. Idles fine and driveability is still great, its just not as strong.

Since the computer runs in a closed rich loop when warming up and then changes after it is fully warm I felt that it was lean after warm up hence it did not feel as strong so I thought maybe the pressure was set wrong. To my surprise it is 51 psi right where TPIS says it works well for my combination.

Question: What do you all thing, should I bump the pressure higher to like 55 or 58 psi? Should I look at replaceing items in the open loop like the TPS or the MAP sensor or the maifold temp sensor? I dont have any fault codes. This is speed denisty so I would think the manual fuel pressre regulator would be useful.

Im confused as to why it isnt as strong after warm up and where i really should have my fuel pressure set at.

Thanks to all in advance for sitting through a very long post.

-Bill

 

Get Datamaster and do a scan with it.There is no sense throwing money at it without some good diagnostics.Could be a lot of things.Normal maintenence and cleaning of the TB would be a good start to eleminate any clogged passages there.When was the last full tune up?

 

It sounds to me that you need to analyze it with the aid of an A/F meter. Until it goes into closed loop, the car is running off of preprogrammed pulse widths. Increased fuel pressure will richen the mixture and because the O2 isn't working yet. The mixture is not controlled. It works the same way at WOT. The ECM is preprogrammed with settings for the Power Enhancement (PE) mode. Only while cruising is the ECM regulating the A/F ratio. Once you take some readings to find out what the engine likes, you can start manipulating the mixtures to give you what you want.

RACE ON!!!

 

Last tune up ( plugs, plug wires, cap, rotor, fuel filter, fully cleaned TB and bypass valve etc ) was few years back. I only put a few thousand miles a year on the car as I only drive it on nice days and it is stored in the winter in my garage. I dont have any fault codes at present.

 

So at part throttle driving around, it does not matter what I have the fuel pressure set at? This is true for Speed Density as well as MAF vettes? Thanks

 

Within the range in which the ECM and injectors can compensate, the fuel pressure shouldn't matter, much. The fuel mixture is mostly tuned by the feed back from the O2. Whether MAF or MAP should be of little consequence.

RACE ON!!!

 

I would look at your vacuum hoses too.Have you checked your vacuum pressure @ idle.Mine is 20inHg.You can try to disconnect the vacuum from the EGR and plug it for a test of the EGR mine was loosing vacuum there.Look at all the easy stuff first .Milage doesn't matter with this kind of problem it can occur at any time.Check the back of the TB plates if they are black do the TB cleaning.