Fuel pressure and knock

[Lhorn]

I've been getting knock at or under 10lb boost. Put a fuel pressure gauge on and was getting only about 30psi with the stock pump /pressure regulator. I figured the pump was worn out so I replaced with a Walbro 340 (not hotwired yet) and a used adjustable fuel pressure regulator. I still seem to be getting knock at/under 10lb boost. What am I doing wrong?

I set the FPR at 42psi at idle (with the vacuum disconnected and plugged). When I hook the vacuum up, the fuel pressure at idle drops to about 32-34. It does seem to go up with some boost (to perhaps 42 or so at 10lbs boost). Should the pressure drop like this when the vacuum is hooked up? Am I still setting this thing too low? Should I bump the pressure until it is about 40ish at idle with the vacuum connected?

Car is basically STOCK.

 

[Quick6'n'-K.C.]

The fuel pressure dropping when you apply the vacuum hose is typical. Lets just hope that it rises when the boost is applied.

Check all your hoses for vacuum leaks,splits,cracks ect. replace them if they are stock originals. 7/32 hose at your local napa.

You should hotwire the pump for more voltage, this will help some, and increase your fuel pressure too, you will need to readjust once you do the hotwire kit.

Replace your fuel filter.

Might be getting knock because of octane, are you running 87 or 91?

BW

 

[Lhorn]

I believe it's 89 octane. The fuel filter has less than 200 miles on it. The hotwire kit is in the mail.

When the vacuum is hooked up, what kind of fuel pressure should I see at idle?

Most of the hoses are new. I just isolated a vacuum leak and I think I'm now ok from that standpoint.

 

[WHEEZER]

Originally posted by Lhorn
I believe it's 89 octane. The fuel filter has less than 200 miles on it. The hotwire kit is in the mail.

When the vacuum is hooked up, what kind of fuel pressure should I see at idle?

Most of the hoses are new. I just isolated a vacuum leak and I think I'm now ok from that standpoint.

Up your octane. These cars are designed to need a decent octane number from the factory, even on your fuel gauge it says premium unleaded only. JMO.

 

[Quick6'n'-K.C.]

Well, i usually run 87 octane.

The only time my car would knock at moderate throttle was during a long steady hill climb with a Eric Marshall chip running about 6-9psi of boost for 20 seconds or so. My scanmaster would register 3-5° of knock retard with the pedal at a steady pace.

I sure wished i knew the incline °, but i dont. Semi trucks eventually slow down to 45-55MPH.

My alky kicks in at 12psi. I use a copy of Ed Brewers Race/Alky chip in my daily driver.

I would suspect that your car has a octane/vs/timing issue.

what chip do you have?
Does the knock go away when you floor it?
Do you have stock injectors?

You will see base fuel pressure, ie 42 line off, minus vacuum, or there close, 15 inches of vacuum is normal, close to 30 is fine IMO.

How many miles on the car?

BW

 

[Mightysix]

Bump the octane you are using. Pump gas sux to begin with..

Mike

 

[Lhorn]

It's about 140K miles on the car, but only a few hundred on a stock rebuilt engine. I believe the chip is stock. The injectors are stock, but were sent out to a guy on this board for cleaning when the engine was rebuilt recently.

I haven't tried to floor it, because I'm afraid of blowing a head gasket if it's knocking. I basically keep pushing a accelerator more until I get knock. Once I got knock real early and once I got up to about 12 lbs, but generally I start to get a blip on the knock guage at 10lbs.

I'll probably put some 91 octane (the best we can get out here) and turn up the fuel pressure a pound of two and try again in the morning. Any other thoughts? I don't want to start messing with new chips etc until I get the basics sorted out.

 

[WHEEZER]

Originally posted by Lhorn
It's about 140K miles on the car, but only a few hundred on a stock rebuilt engine. I believe the chip is stock. The injectors are stock, but were sent out to a guy on this board for cleaning when the engine was rebuilt recently.

I haven't tried to floor it, because I'm afraid of blowing a head gasket if it's knocking. I basically keep pushing a accelerator more until I get knock. Once I got knock real early and once I got up to about 12 lbs, but generally I start to get a blip on the knock guage at 10lbs.

I'll probably put some 91 octane (the best we can get out here) and turn up the fuel pressure a pound of two and try again in the morning. Any other thoughts? I don't want to start messing with new chips etc until I get the basics sorted out.

Your chip and injectors need to be VERIFIED. That could be all the problem.

 

[WHEEZER]

Originally posted by Mightysix
Bump the octane you are using. Pump gas sux to begin with..

Mike

Ain't that the truth..don't even have a decent smell anymore. Smells like crap.

 

[Lhorn]

Here's a stupid question: How do I know if it's a stock chip? Any special markings?

 

[Mightysix]

Not a stupid question. Mine was all black and some GM gibber on it.

HTH's Mike

 

[Lhorn]

Looks to me like a stock chip. It's black with "Delco" and some other numbers printed on it.

Tried to fill up with 91 octane and still start getting blips on the knock guage at about 8-11 pounds boost.

Doesn't seem to happen at shift points. Should I put a few gallons of race gas in and see it still happens? Any other things I should try?

Maybe I should get DS.

Thanks.

 

[Mightysix]

Race Gas???? Hell yeah....