Something is happening that is causing the pressure to drop. When it drops below 50psi, there just isn't pressure enough to properly open the poppet nozzles. That is unless you have the MFI spider in there...

You will have a ground, a +5V reference, and the sensor return. Turn the ignition on and test the wiring to determine which is which. Back probe the sensor return and plug the connector back into the TPS sensor. Slowly actuate the throttle from closed to fully open and watch the voltage.

 

MFI--Multi-port Fuel Injection?

If that is what you are talking about, I read somewhere if replacing the spider replace with MFI. I have the original spider. Could this spider have a sticking poppet or 2 causing the reduced fuel pressure? Even if not, if I replace the spider will the new spider work at lower pressures?

I really appreciate you taking the time to help me.

A/M/G

I just checked ths TPS. I read .507 volts throttle closed. and .567 full open. No dead spots. I assume the TPS is bad. It is susposed to go from .500 closed to 4.5 volts fully open correct?

 

Going from the SCFI spider to the MFI spider will remove the problematic poppet nozzles and move the electronic injectors out to the intake runners. Since the poppet nozzles open based on pressure which is fed from the electronic injectors in the injector pack on the SCFI spider, the line pressure must be sufficiently high enough (>52psi) to open them.

The MFI spider will spray fuel whenever the electronic injector pulse tells them to. However, if the fuel pressure is not high enough, it will not properly atomize the fuel so the vehicle may run poorly at low pressures. If the pressure is too low, the truck won't run.

I would recheck your TPS sensor again just to be sure that those numbers are accurate. That sounds quite bad...

 

I checked the TPS 2 or 3 times. I got my 5 volt reference and my .5 volt sensor return everytime. Replaced the TPS, no help. The new TPS .5 volts to 1.25 volts, that is all, not 4.5 volts.

With the fuel press gage on again. I watched VERY closely, when faulting.
1. FP is at 51-52 psi, it is normally about 53-54 after coming off the throttle
2. going into throttle, FP starts to rise to 53, eng start to increase in RPM to about 1500
3. Fp starts falling, falls to 44-40, eng stumbles, backfires, bucks
4. release throttle, Fp increases to 51-52, truck idles
5. repeat process
6. turn key off and back on 62 psi immediately, runs fine until FP fall to 51-52 again.


What do you mean "move the electronic injectors out to the intake runner"?

If I put a MFI in the truck, is it simply remove the entire spider and install the new spider, everthing plugs right back in?

I really think either the FPR or the spider is bad. I have replaced the FPR, it was an after market regulator, it was leaking would not hold pressure per the DIY section.

I thought maybe I was losing vacuum in the plenum , rechecked my work of the plenum installation after FPR replacement, all good, cannot hear any vacuum leaks. Removed the purge solenoid valve and plugged the hole in the plenum, no help. I thought when it was opening maybe it was sticking open.

Again, I really appreciated your insight and your time helping me with this. I have been trying to figure this thing out for over 3 months now. If I cannot figure it out I will have to buy another truck. It really is not in the budget. The truck has less the 90k on it, once this is done I think it will give good service.

Thanks
A/M/G

 

Your TPS readings make no sense what so ever. I would check the sensor ground to make sure that it is reading properly the whole time.

As far as your fuel pressure, I still think that it is a supply problem.

Read up on the MFI spider upgrade. I have posted all of it before so there is no sense in repeating things. "MFI+upgrade" as the search term and "swartlkk" as the author should turn up the pertinent results (with out the quotes).

You may want to pull the fuel filter and get the adapters to connect your fuel pressure gauge at that location. Your gauge should have a relieve on it that will allow you to test not only the static pressure that the pump can generate, but also some sense of how well the pump can provide that pressure as it is flowing fuel. Some fuel gauges do not have all that large of a relieve valve on them though... The fuel pressure test port is a JIC fitting. This is another one of those things I have detailed in other posts so you should be able to search for "JIC" and "swartlkk" as the author and find the information.

 

OK, I bought the MFI part #12568332 from GMPD.COM. I thought it came with the seals and and the bracket, no seals no bracket. Do I really need the new bracket, or will the MFI install directly on the old bracket? I know,I will need a new plenum gasket set. I will buy them from a local auto store. Anything else I may need? I do not want to open the engine and find out I need additional parts.

Thanks A/M/G

 

just for the heck of it....how long ago was the last pump put in it and what was the brand name of it? Jerry

 

Delphi, maybe 3-45 days.

 

You shouldn't need to replace the plenum gasket unless it is torn or nicked. Being an o-ring style gasket, you can remove it, clean it up, and check it for signs of problems. If it checks out good, reuse it. Same goes for the spider to upper plenum seal.

You will not need the bracket.

As far as the MFI spider coming with the bracket and gaskets, it all depends on what you ordered. There is a 'kit' available that will come with these items, but there also is just the spider.

 

Thanks, I just ordered the MFI, but I thought it was the kit. I just replaced the PFR a couple of months ago so I can reuse the orings. The bracket not needed, good deal.

This weekend I will replace the spider.

Thanks
A/M/G