I have a 91 5.7 tpi trans am. 3 years ago I replaced the fuel pressure regular when an injector went bad and also replaced the fuel pump and pulsator with a piece of fuel injection hose. About a year ago the car developed a hard start problem and I traced it back to not having any fuel pressure at start up. I installed a tester gauge on the fuel rail, and noted pressure engine running cold was about 40 psi, I removed the vac hose from the regulator and psi went to about 50. Next test was with engine stopped key on, the fuel pressure went up to about 45 psi and bled to zero just as soon as the fuel pump stopped. The next thing I did was clamped off the return hose, key on engine stopped, the fuel pressure went up to almost 70 and very very slowly bled back down. Does this sound like a bad fuel pressure regulator? Even though I replaced it about 3 years ago? Any info would be helpful, thanks.

 

yes. the diaphragm more than likely has failed. I was gonna say that you may have a bad/leaky injector. But since you noted that you pinched off the return line, that cancels out the bad injector.

 

You can also pull the vacline and take a good wiff. If it has a strong gasoline smell, the diaphragm is bad.

 

Thanks to both replies. Theres no trace of gas or gas smell in the vac line of the fpr.

 

WHAT!!! OOOOO. Heck man. I'm bamboozled.

 

Yeah its really weird and I have read about others who had the same prob. Replacing the fuel pump, injectors, fpr, pulsator, and replacing them more than once and still having the same prob. If they ever figured out the problem they never posted but if I do I will certainly.

 

Yeah man. That happens a lot. They come for help. Once they get the help, they disapeer. Hopefully your determination pays off and you get the issue fixed. As you can see, not very many people are willing to lend a hand. Someone who could probably give you more help is a member named vetteoz. Look for him in the TPI threads.

 

Ok thank you

 

No problem man. Good luck.

 

If you are sure the diaphragm is good then a possible cause of a pressure leak down through the reg
( proven to be the leak source by the pressure holding when the return line was clamped )
are a bad spring in the reg not holding the seal on the seat correctly or a bad / damaged seat letting the pressure bleed off ?

 

Hey guys. I have found the problem. After much research, someone with the same problem said that putting a resistor on the coolant sensor wire to the ecm will fix the problem. This struck me as kind of odd, since of course the car didnt come from the factory with a resistor on the cts. That got me thinking about the wiring and since I know how hokey the wiring can be on these cars and the way it can cause problems with the readings on sensors when it degrades. I never really knew though how important the cts was to the overall running/starting of the car. Anyway, I replaced the cts connector and the two wires from it all the way to the ecm. The car starts perfectly now, stone cold or warm. I think if more people who are having hard start issues tries this, they will be surprised like I was. Thanks everybody for the suggestions and help!

 

Wow. Strange but congrats man.

 

Witch wire, witch cts, how many ohms

 

 

None of this has anything to do with your original reported problem; loss of fuel pressure on start up
The ECM does not control the fuel pressure

Replacing the CTS as you did is the only correct fix given the engine temp ( from the CTS ) is the only way the ECM knows what fuel to supply
A resistor is only a band aid to a correctly functioning CTS . fooling the ECM into thinking the engine is at a temp it is not

 

Tpi has 3 cts and I don't see how it fixes it that's why I wanted to know more. And the search on here sucks, I have a hard time looking for my stuff just to see if it comes up. Google is way better and I was talking to the OP

 

So was I

TPI has only ONE CTS ;(Coolant Temp Sensor ) ;the 2 wire unit at the front of the intake circled in pic below.

Anything else ( most likely a 1 wire unit ???? ) in the coolant you might THINK is a CTS is either a sensor for the temp gauge or a fan switch.
Neither of which go to the ECM to indicate engine temp

 

The TPI has 3 CTS's?

 

I thight the one next to it was one too, they all may do different task but they are CTS like the one on the side of the head. But if I'm wrong I'm sorry school me. Is the one next to it for the fans

 

Nope. You got it all wrong. There is the CTS/Coolant Temperature Sensor (tells the ECM the coolant temperature). There is the CTS/Coolant Temperature Sender (for temp gauge). And there is the CST Switch/Cold Start Temp Switch. So they are not like one an other.

And please don't say the MAT Sensor is the same just because it looks similar.

 

Also the one on the head is way different than the others. It only has one wire while the others have 2.

 

What I mean they do the same thing in away is that they all read coolant temps but for different tasks.

 

Not really .
The CTS and the gauge sender unit have a resistance that varies with temp
The fan and the cold start switches are just that ; On/off switches that operate at a certain temp.

Calling them all a CTS is just going to confuse people if you are looking for help.
The CTS is for the ECM ;2 wire
The temp gauge sender unit ;1 wire
The fan switch ;1 wire
The CS switch ;2 wire

All have different connectors so it is impossible to confuse one with another

 

This is exactly correct.

But since it was mentioned, the Air Intake Temperature sensor in the plenum (which does not touch water), is the same sensor and connector type as is used for the CTS.

 

Ok see u helped me thank you. I Have a Corvette base and it doesn't have the fan switch CTS. Can I just splice in a therm the ones for rads, like if u have a aftermarket fan set up. U get what I'm saying.

 

Just so that members don't get confused, that AIT/MAT SENSOR records the plenums heat more than the plenums air temperature due to the plenums heat soak and the sensors all metal body.

 

You are still doing it

It is not a CTS ; it is a FAN SWITCH

No base has a fan switch ; only the REAL CTS and the CS switch ;the switch for the fan is in the head
If the switch that controls your fan is faulty, you just replace it