Unknown facts regarding replacement 1st gen. Vortec MP fuel systems

[speedygonzales]

This information is about changing the crappy CSFI spider to the much better MP spider.

We've all seen them from different companies. GM, AC Delco, BWD etc. They are all manufactured by Standard Motor Products.

Here's an example. MP replacement

But 2 things I bet you did NOT know is: What pressure they should achieve and that they can NOT achieve that pressure.

Since I rebuilt my engine a 1 1/2 yrs ago, I never checked the pressure of system. It has always idled crappy (lets say crappier then before). Last month I installed a gauge and confirmed it's reading with a Snap-on gauge. The unit from the build could not achieve 45 pounds. Now first remember the OEM spec is 60 to 66 pounds.

So I first changed the FPR no change. Then changed the fuel pump, no change.

So I surmised the only thing it could be was the central fuel block. I ordered a new injector unit and returned the old one for credit. The pressure now was higher but still not spec. New one was still only 48 pounds.

I also figured that due to the fact that both units had low pressure, the specification must have changed. Contacted Standard in Virginia and found out the MP spec was 55 to 62 pounds. But these units still could not get there. So an easy mod to the return line and it runs 100 percent better.

So if you don't like the way the vehicle idles after the install of the MP unit, pressure is the problem. Now that I've upped the pressure to spec, it idles smooth as silk.

Summary for those searching on the internet for information regarding the Vortec MP injector unit.

The new pressure spec is 55 to 62 pounds. Chances are good the unit you get can't achieve it. Modification to the return line is the only way to ensure you get the pressure correct.

You can either slightly crimp the return line (near the schrader valve is easiest). Or Try this to correct it. fastenal no.68779 since the return line is 5/16 and most do it centers don't carry that size. Fastenal ships to your door in 3 days.

I hope this information helps someone.

[OrangeAnimal]

There is no available adjustable pressure regulators on the market? I assume you may have looked but just curious.

Also I wonder if you could have issues with the pulse damper. It's inline on some cars and some it's built into the pump unit itself. Just as the name presumes it smooths out the flow of fuel from the pump since all rotary pumps don't actually deliver a steady flow under pressure. I have seen them become clogged with crap of years of use.

I can't remember exactly but can you or did you disconnect the vacuum supply to the regulator when you checked the pressure. Did it go up any? It should have jumped to about 65psi max. Atleast that's the highest I have ever seen one with a stock pump achieve.

[speedygonzales]

There are no adjustable type cause it's under the upper plenum shown here
http://s207.photobucket.com/user/twi...ntake.jpg.html

I changed the pump so since nothing changed there, they weren't the problem.
Stock spec is 60 to 66

Something else I found out is the pressure drops over night but never gets to 0. So the fuel is always at the rails.

[67c10rustbuket]

Good info, I will be removing my intake to see if my engine has the old injectors or the replacement soon. I have a boyds tank with aeromotive pump so I think my pressure should be fine. (In a 67 C10 running 97 Tahoe vortec csfi? engine)

[speedygonzales]

The pump makes no difference. The new injector fuel block is lowering the pressure below spec. 55 to 62 PSI. If you replace the injectors with the new style, check the pressure before you fire it up.

[Tom]

Exellent info speedy!!! From what I read, your actually trying to reduce the return line flow rate so as to up the fuel pressure correct?

[speedygonzales]

Yes, I have pinched the return line to reduce it's size temporarily until I can get a shutoff valve inline. The size is not a common hardware store item and I did not want one of those 1/4 or 1/2 turn ball valves. They would be too hard to dial in the correct pressure.

The one thing that concerns me with the gate valve I did find is if the packing will stand up to fuel and ethanol.

[TennesseeZ]

Speedy, not trying to pick hairs here, but going from 35 yrs as a pipefitter, I believe you're linking to a needle valve in your first post, not a gate valve. A needle valve, or preferredly a globe valve, of which you'll probably not find in 5/16, can be used to fairly accurately regulate flow. The gate valve is not really a "fine adjustment" device. Gate valves, and ball valves, are the most common types available.




































































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[speedygonzales]

So I used the wrong term. Long as it's not a ball valve, it will still give finer adjustment.

[Rufton]

99 350 vortec fuel pressure was 58psi w/ OEM spider.
I swapped spiders (see pics) and it runs perfect.

[speedygonzales]

58 is below spec for OEM. The replacement will be even further away from it's new spec of 55 -62. I would put money on it.

Also I thought mine ran ok until I brought the pressure up to the new spec for the new unit also.

[Rufton]

Both spider styles have built in pressure regulator to control injector pressure. Don't follow why spider specified for OEM application would require a restricted return line.

[speedygonzales]

Originally Posted by Rufton

Don't follow why spider specified for OEM application would require a restricted return line.

Originally Posted by speedygonzales

they can NOT achieve that pressure.

The unit from the build could not achieve 45 pounds. Now first remember the OEM spec is 60 to 66 pounds.

So I first changed the FPR no change. Then changed the fuel pump, no change.

So I surmised the only thing it could be was the central fuel block. I ordered a new injector unit and returned the old one for credit. The pressure now was higher but still not spec. New one was still only 48 pounds.

MP spec was 55 to 62 pounds. But these units still could not get there. So an easy mod to the return line and it runs 100 percent better.

In short the original spec (using the old poppet style injectors) for fuel pressure is NOT 58 PSI it is 60 to 66 PSI. I have the GM shop manual.

Second, the replacement MP unit has a lower spec. It is 55 to 62. I checked with Standard. (Standard is the manufacturer contracted by GM). This lower spec for the replacement MP unit is not printed anywhere.

Third, 2 different units purchased 1 1/2 years apart could NOT obtain the 55 pounds minimum. Once I brought pressure up to spec, it ran much better. (for those that did not read, I first changed the FPR then I changed the pump).

For those that did change to the new style injectors, even if you think it runs ok, check the fuel pressure. I an willing to bet that it does not get to 55 PSI minimum and if the pressure of your unit is upped to spec, it will run even better.

That is why I upped the pressure.

[Rufton]

Originally Posted by speedygonzales

That is why I upped the pressure.

I understand why you crimped your return line, but isn't there is a possibility that something is out of spec on your vehicle? Maybe your fuel supply line pressure differential, amperage available to your pump, or other gremlin has caused a problem specific to your vehicle. It would be nice to see more than one data point.

[Tom]

Originally Posted by Rufton

It would be nice to see more than one data point.

You want data points when all he's said is this is his experience, he thinks others will experience the same thing, and that its something worth checking if you do the conversion....

[Rufton]

Everyone likely knows but I'd like to reiterate safety first if decide to splice valve into high pressure fuel line.