Guys, I'm having a hell of a time with this cold start problem on my 95 Z-28. It also has a part throttle miss, but I haven't even been able to drive it since its a pain to get it running.

I have another 3 page thread about it, but the only person posting in the thread at this point is myself.

Background on the car:

New OPTI, Water temp sensor, plugs, wires, ICM ( with the proper paste behind it), Coil, Oxygen Sensors, IGN Switch on column, Water Pump (electric), and a Racetronix FP kit (harness not yet installed) thats about 4 months old.

Per someone else's suggestion, I left my fuel pressure gauge hooked up overnight. It bled down to 0. When I pull the vacuum plug on the regulator it doesn't have any fuel in it.

I finally was able to have someone help me this morning, and I ran a wire from the pink wire on the coil, to observe the voltage of the IGN while the motor is cranking. This way I can also provide the wire with a direct +12v while it is cranking to see if I get an immediate start up. The voltage did drop while cranking down to 9.6 volts, but it was hard to get a steady reading with the numbers jumping around.

Well, it started on the second cycle of the key, when I touched the wire to the +BATT. It didn't fire right away, but it did fire up with my girl cranking it, and no coaxing/throttle or anything.

Once it was running, the pressure at idle was a little lower than usual, like 36-37 psi, and when I had her bring the RPM up to 2000, it dropped all the way down to like 34, 3000 RPM was about the same maybe a little lower.

This was the first time I had some help to do this, as before I was just removing the vacuum line from the regulator and the PSI was shooting up to 45psi+ like it should.

Bad regulator? Should I go with an aftermarket stock replacement (local parts store), or an aftermarket performance unit?

Even if the regulator was bad, sounds like the new Walbro is shot too?

Any help is appreciated.

 

I don't know what the fuel pressure is supposed to be at idle with the vacumn line connected but it sounds like your 36-37 is right cause I know it's suppose to be 43.5 with the vacumn line disconected (which simulates WOT). But it doesn't make sense that when you said you had her bring the throttle up to 2,000 the fuel pressure dropped to 34... The higher the rpm the higher the fuel pressure is going to be. If you hook the gauge up and check the pressure (car off, priming it) , will it hold the pressure or does it bleed off?

 

It doesn't bleed down right away, but it did bleed down overnight - about 12 hours. Tonite I am going to prime it, then monitor it every hour to see how fast the pressure bleeds off.

I know it doesn't make sense. With the vac line disconnected it pushes 45-46 PSI. Idle is 37-38 PSI. Then she brings it up to 2000 rpm and its at 34. 3000 rpm its 32-33. WTF?

 

Just was ready and apparently Racetronix says that a Walbro pump requires TWICE as much power to run as a stocker?

Guess I should install the harness that came with it ASAP!

Why don't they put a disclaimer on the pump that says that the pump will either die or not perform right without an upgraded harness?

 

Ya eventually your going to lose your pressure, It least it is holding it though... The thing that doesn't make sense is it's losing pressure as it's gaining in rpm... So What would happen if you brought the rpm up to lets say 4000 to 5000? does the pressure come back? It has to eventually because your saying that it's getting 45 to 46 with the vacumn line disconnected... I guess install your new harness and report back what happens

 

What you're explaining is NORMAL. With the vacuum line off the regulator, it's not receiving any vacuum (obviously) and is at it's max pressure. Vacuum applied to the regulator drops the pressure down. This is why you see a lower fuel pressure at idle, and even lower pressure revving the motor, because you're pulling more vacuum through the motor.

It's normal for the pressure of the fuel system to bleed off overnight. Bleeding off in a few minutes is less than normal though. But even with zero pressure, your system should prime for 3 seconds at initial key on. Are you hearing the pump prime?

 

Yes, I am hearing the pump prime every time. It primes and reads about 39-40 psi prior to cranking.

I thought that I should see an increase in fuel PSI with an increase in engine RPM? SO it will only spike the pressure when you BLIP the throttle or punch it? I wasn't having her "REV" the motor, but rather bring the motor to 2000 RPM and hold it steady, then 3000.

What kind of PSI should I see at 2000 or 3000 rpm?

 

I understand what your saying with the vacuum line off it will have max fuel psi reading, and I understand what your saying that at idle the manifold vacuum is going to lower the fuel pressure... But shouldn't the fuel pressure read the lowest when he revs it and for a split second when the throttle plate closes it will read the lowest ? And when he's getting into the throttle the manifold vacuum is dropping thus increasing the fuel pressure? Correct me if i'm wrong but that's how I thought it worked... The only thing that gets me is he's saying that he isn't revving the motor, but slowly bringing it up and holding it at a given rpm so if he's holding it at lets say 2000 or even 3000 rpm steadily, that fuel pressure should be bumping up shouldn't it? And if he was revving it it would increase then decrease very fast right? Let us know what you come up with.. The racetronix thing might help you, If the racetronix thing has a step to step diagnosis procedure i'm sure you'll figure out what your issue is...

 

Instead of shopping for opinions perhaps you should follow the GM diagnostic procedure posted in our support forum. If you use the GM manual without skipping any steps you will have an accurate diagnosis!

 

Thanks, I will look into that.

I also had a separate problem which may or may not be realted, which is why I am "shopping" for opinions.

But thanks, I will use that info from your site to make an accurate diagnosis.

Side note - how often do your (Walbro) pumps go out using the stock wiring set-up? Are the pumps not meant to be used with factory wiring at all?

 

hmmm well i'll chime in lol...i've had mine wired in with the factory wire and had zero problems with mine and it's been in there 7+ years....

 

SO I registered on your site, and can't find what you are talking about. Matter of fact its an empty forum. I can't see any threads or anything.

Also did a search on the site for "GM diagnostic procedure" and had no luck.

I also PM'ed you and haven't heard back.

Does anyone else have acess to the GM manual and steps to properly diagnose this fuel system?

EDIT: guess they must not have activated my account yet at that time, I found it.