: Fuel Pump Advice sought - 96 STS

10-11-04, 06:30 AM
For some time now my '96 STS has required considerable cranking to get it to fire after sitting overnight. The explanation was that there is an anit-back flow valve in the fuel sending unit that often fails, allowing the fuel to siphon back into the tank out of the fuel lines. Does this sound correct?

If so, how difficult is it to remove the fuel tank and replace the fuel pump? Is it something a guy can do in his garage or better left to experts?

10-11-04, 09:52 AM
Have you checked your fuel pressure regulator? Leaky FPR's can cause a hard to start problem, Pull your vacuum hose off the FPR and see if any gas is dripping out of the vacuum connection, and also check the fuel pressure by hooking a fuel pressure gauge to the scrader connection on the fuel rail.

10-11-04, 04:04 PM
I own a í97 SLS and as of yet havenít had that problem.

I did have a similar problem on my chevy truck. It turned out to be a bad fuel pump relay. The way I understand it during startup the fuel pump relay sends voltage to the fuel pump for two to three seconds. This supplies fuel pressure to start the engine. When the engine starts oil pressure builds allowing the electrical circuit to be completed through the oil pressure-sending unit (switch, sensor or whatever the current term for that device) and on to the fuel pump. When the truck was cold in the morning it would take the longest to start. I would crank the engine for maybe 30 seconds to a minute, watch the oil light on the dash go out and within just a few seconds the truck would start. The Cadillac may have other circuits involved but my truck was wired that way. This was explained to me to be a safety measure. If the vehicle was in a crash, the engine had stopped, the ignition was still on and a fuel line had ruptured there would be less fire danger. In other words the engine being stopped no longer created oil pressure to the oil pressure sensor which no longer allowed voltage to the fuel pump. No fuel under pressure running out on the ground or hot parts on the engine.

Maybe the Cadillac is wired differently? I am sure if I am wrong someone will correct me PDQ. Maybe using my truck is a bad example but that fixed my truck.

10-11-04, 05:42 PM
Id check the FPR too... I didnt know about that, haymaker... Thats a neat feature, it wont start until adequate oil pressure.....

10-11-04, 06:09 PM
You can easily check the valve by putting a fuel pressure gauge on the rail, and seeing how long it holds pressure. If it drops off like a rock, it probably is the valve... or a leaking injector. As to how easy it is to drop the tank... It's fairly straightforward. I think I did it in an hour or so... but this is from my parts car, and I didn't care to save it, so I cut the fuel lines. To this day, I still don't know how you're supposed to remove them.

10-11-04, 08:03 PM
Thanks for your advice so far. I should have mentioned that I had already replaced the fuel pressure regulator as an inexpensive first step. It showed a drip, but replacing it did nothing for the hard starting problem.

10-11-04, 10:08 PM
You really need to put a fuel pressure gauge on the rail and monitor the pressure during crank.

The fuel pump has a valve in it that is supposed to hold pressure during shutdown...but that is for hot fuel handling concerns. Even if the valve is allowing the system to leak down it should not affect the cold starts. If the cold start is the problem then checking the fuel pressure to determine if it is slowling coming up to pressure...or delaying and then jumping instantly to full pressure will give you a clue as to where to look next.

If the fuel pump is getting weak (starting to short out windings in the armature) it will start to draw more and more current. This causes the pump performance to fall off drastically when the battery voltage is low...like during a cold crank. That would be indicated by a slow but steady buildup in pressure during the cold crank.

If you put the gauge on and see no pressure for the first part of the crank and then the pressure suddenly jumps up to full pressure then I would check the fuel pump relay and relay circuit. The pump circuit is fed redundantly thru the oil pressure switch so that you will eventually get pump voltage even if the fuel pump relay (controlled by the PCM) is failed completely. This would typically cause a long crank until some oil pressure showed up and then the pump would start running. You can also check this on the next cold start by removing the fuel cap, putting your ear to the fuel neck and listening for the pump to run momentarily when the key is turned to "ON" and not cranked. You should be able to hear the pump run in the tank for 2 to 3 seconds when the PCM turns on the pump to prime the system. If you cannot hear it then check the fuel pump relay.

It is quick and easy to put a fuel pressure gauge on the rail as there is a screw on schraeder fitting on the rail for that purpose.

10-24-05, 10:36 PM
We have a 98 Catera it won't start immeidatelly. I have to wait like 30 secs. or so to hear a little revving and then a click, does it have to do with the fuel pump? my guess is yes. If I want to start the car right away it will crank but won't start, I realized that I need to wait untill I hear that click.

What do you suggest.

10-24-05, 11:16 PM
Check the fuel pump relay. If it is bad, the pump won't energize til you build some oil pressure while cranking and that takes a little time.