1990 fuel pressure & vac readings good/bad?

[biggiefl]

OK....1990 stock ZR1 #451 Just put in 16 new injectors a few weeks ago(Bosch). Went through all the vac. lines and all seemed good but I did not pressure test while plenum off. Runs and idles great now. So......:

1) at idle I have 42psi pressure on fuel rail. With engine off it drops to like 39 and stays. Will hit under full throttle like 48psi but at around 4800-5k rpm it drops sharply to around 30psi.

2) Vacuum gauge run off the drivers side center vac line going into the plenum(between #3 & 5 cyl. Read 12nm or whatever at idle. Would drop close to 0 when throttle blipped and up close to 25 and settle down to 12 again, no 17-22 normal. Brand new gauge and might not be accurate(will test on another car)but steady at 12. I think I might need to use a manifold vac line, not plenum but read that was the place? Some say start by vac pump or vac line under ECM?

3) I have used cold and warm engine temp. and sprayed ether around the entire engine, no pick up in idle.....nuttin unless sprayed in the air filter. Sprayed around every line and pump....nuttin. Could be I need to be under throttle to check some of the secondary stuff.

I'm thinking fuel pump(s) or filters or both......thoughts?

I'm getting tired(literally my back) with messing with this and would appreciate any and all ideas.

I know the info. is here but if you can supply part/pump info. #,s from rock auto or just the car/model(96 454 suburban?) would be mucho helpful. I am getting close but not sure where to start. I think the 30psi at 5k is not good, some say 55psi. I have more than 210hp but nothing close to 375 if that helps. I can get over 6k but it is not screaming like it did.

PS...As per rec on Mark's site I put in new AC Delco RF-5 plugs at I think .35 gap. Was that incorrect? Again runs and idles awesome, just not full power. I've heard .42 instead of .50 is good....confused.

Thanks, Nick

[Jitse]

I suggest to change the fuel filter, and test again. When this is not the solution go further with the fuel pumps checks.

[Marc Haibeck]

The fuel pressure drop off that you are describing is typical for a secondary fuel pump that is not working. The fuel pressure is normally 48 to 51 PSI from 3000 to 7000 rpm at wide open throttle. When driving the car you should see the fuel pressure spike up a few PSI when the secondary pump turns on at 4000 rpm.

An easy way to test the pumps is to open the electrical connector to the pumps at the fuel tank. Remove the fuel door panel to access it. Apply 12 volts through a 5 amp fuse between the black and grey wires. This should run the primary pump. Connect the 12 volts between the black and the green wire. This will run the secondary pump. Never connect to the purple wire. Doing so will damage the fuel level sensor. If the secondary pump runs, check the fuse under the right hush panel.

We use the Airtex E3240 as a replacement pump.

A .035" gap is good for Rapid Fire spark plugs.

[biggiefl]

Thanks guys. Should i be worried about the vacuum numbers?

[Marc Haibeck]

12 inches of vacuum at idle is normal for a LT5 with stock cams. You should have close to 29 inches at wide open throttle. Your 25 inches makes me wonder if the air filter dirty. Or, maybe the low fuel pressure and low power is preventing the engine from pumping normally.

[biggiefl]

Thank Mr. Haibeck for commenting on my post. I am truly honored.:saluting:

The 25 on vac. Was just a throttle blip, not wot. Air filter is brand new along with a new battery, injectors, etc. Ordered new airtex e3240 pump kits, strainers and new fuel filters. Should be 100%.

Thanks to everyone for their help. After 2 months....i think i got this😎