1998 Bravada fuel pump issues


realdeal8471
07-12-2009, 09:13 PM
First off, I want to say thanks for all the useful information here. I have been visiting this site for years, and finally decided to join. I hope some of you can help me figure this out. Here is my dilemma:

Ex-wife's Bravada has 137k miles, and has had the fuel pump changed three times in the last year. Still has the same problem. During the hot months (I live in central Arkansas), shortly after reaching operating temperature, the truck will die and not re-start. After it sits for anywhere from 10 minutes to 2 hours, it will start back up and run like a new one. Also, a couple of minutes before it shuts down, the air conditioner quits blowing cold. Just going by info that she had told me, I had assumed that the fuel pump was just shutting completely off. During a no start situation, it will start and run for a few seconds if starting fluid is sprayed in the throttle body. I had her bring it to me this weekend so I could try and diagnose it. I checked fuel pressure key on engine off, had about 63lbs. Started it up, and went driving up and down my street armed with all my tools and electrical testing equipment. After 15 minutes or so, the air quit blowing cold, couple minutes later it dies. The first thing I check is the fuel pressure, and it is about 45lbs. The pump will run, but obviously not enough pressure. So my hopes that I was just dealing with a bad ground went up in flames. When the pump shuts off, it does hold pressure. So am I to assume the regulator is OK? I don't know what brand fuel pump has been put in it 3 times. All I can get from her is that it was in an AC/Delco box. She also put a new PCM in it last week thinking that was the problem. Here are my questions:

Is it possible that this is three bad fuel pumps in a row? This problem has stayed the same through out all 3 of them. The pumps were bought at Bumper to Bumper auto parts. I have a '96 Blazer, and I changed my pump a couple of years ago with one from the same store, and no problems since (knocks on wood...).

Also, I noticed when it is running, when I rev the engine, the fuel pressure jumps up. How is that possible with an electric fuel pump? My Blazer does the same thing. I just figured the pressure would stay consistent regardless of engine speed.

I was thinking it may not be getting a full 12 volts during a no start condition, so I applied 12 volts to the fuel pump pin next to the relay, no change in pressure. Is it possible that I have a weak ground problem? Obviously I will check this at the pump itself, but I ran out of daylight, and I am hoping to get some ideas from you guys to check in the coming days.

What is the deal with the AC quitting before it dies? When this happens, the engine is still running strong. Is the ECM sensing a problem and cutting off accessories? This just seems really odd.

Since it holds pressure, am I to assume the regulator is OK, or is that possibly the problem?

Also, I don't understand why this only happens during hot outside temperatures. During cool months or at night, this doesn't seem to happen. Can someone shed some light on this?

She is ready to set this thing on fire and roast marshmallows with it. After this weekend, I am ready to do the same. Am I overlooking something? Any help will be greatly appreciated.

old_master
07-13-2009, 04:14 PM
Key on, engine off, fuel pump running, fuel pressure must be 60psi to 66psi. After the pump shuts off, pressure must remain above 55psi for 3 to 5 minutes. GM does not publish any "engine running" fuel pressure specs, (there are far too many variables involved).

As far as fuel pressure increasing on accelleration: perfectly normal. The fuel comes from the pump somewhere in the neighborhood of 75 to 80psi. It flows past the injectors and up to the fuel pressure regulator. The regulator is inside the plenum and uses a spring loaded diaphragm to regulate fuel pressure. They are adjusted to regulate fuel pressure anywhere from 60psi to 66psi. When fuel pressure overcomes the spring pressure, the excess fuel and pressure are returned to the fuel tank. The regulator has a port that is exposed to the pressure inside the plenum. When the engine is off, the port is exposed to atmospheric pressure and fuel pressure is at its highest, 60psi to 66psi. When the engine is running, vacuum in the plenum pulls against the back side of the diaphragm and slightly compresses the spring in the regulator and fuel pressure decreases. As engine load is increased, vacuum in the plenum decreases and the fuel pressure increases, back up to the 60psi to 66psi range.

If a shot of carb cleaner, (never use starting fluid), in the throttle body gets it started and running, that's a classic symptom of low fuel pressure. 3 fuel pumps in a year is a problem! The most likely cause is a poor ground for the fuel pump causing high resistance which will burn the fuel pump motor prematurely. The fuel pump grounds to the frame rail near the tank and should show less than 5 ohms resistance from the fuel pump connector to ground. Check the fuel pump connector carefully for burnt contacts or melted plastic. Most replacement fuel pumps come with a new connector and pigtail with heavier gauge wires that must be soldered to the original vehicle harness, don't use the crimp connectors that come with the fuel pump. Poor connections will cause low fuel pressure, along with premature pump failure.

The deal with the AC quitting and then the engine, might indicate an ignition switch problem. The ignition switch feeds several different circuits simultaneously and high resistance there could be causing all of your problems. Just something to check. It might help to connect the fuel pressure gauge and drive the vehicle until it acts up, that way you can see what fuel pressure does, and if that's the cause of the stall. Keep us posted.

realdeal8471
07-13-2009, 09:26 PM
Thanks for the reply old master. I replied to your PM as well, thanks for the great info.

Sadly, I had to work late today, so I didn't get to tinker much. But, I did get to check a couple of things. Key on engine off, pump running, I get about 62-63psi. When the pump stops, it drops to about 53-54psi and holds. Running, it stays around 55psi or so at idle. Granted, this is an inexpensive Harbor Freight guage, so I am sure it may be off a couple pounds or so. Thanks for clearing up the issue of the pressure rising when I rev the engine. I had no idea vacuum came into play.

Here are a few things rolling around in my head. First, is it normal for the fuel pump to run for a couple seconds when you turn the key off? I hadn't noticed before, but that is happening.

I know when the pump was changed the SECOND time, the new plug/harness was installed. But I don't know if it was soldered or spliced using the supplied butt connectors. I'll find out. All I know is that he used heat shrink.

After reading your reply, I realized something. When the vehicle quit on me this weekend, I was by myself. So the only way I was getting pressure readings was turning on the key and walking around to look at the guage, which would read about 40-42psi. Since I now know a little more about how this system works, I can't help but wonder if the pump was supplying enough pressure, but the regulator was the culprit, not allowing enough to build up, or is that even possible? I guess the only way to find out is to drive it until it dies again. It's gonna be a hot week, so that should be easy.

I still don't understand how outside temp affects all this.

What you said about the ignition switch has me intrigued. Can you give me some advice on what I should check? The only thing I know for sure is when it dies, I was getting the low pressure reading, and it would start when starting fluid (which I won't be using anymore) or gas is squirted into the throttle body. It will only run for a couple of seconds until that is burned, then back to square one. If it was a problem with the ignition switch, would there be codes? I don't remember if I mentioned it, but there has never been a SES light throughout all of this.

When the original fuel pump went out, it was a classic failure. She just went outside and the vehicle wouldn't start. She called me, and I had her depress the shrader valve after key on, no fuel pressure. She had it changed, all was well for about two weeks. Since then, this problem has been going on through two more fuel pumps. Changing the pump the second and then third time never changed the problem. At all.

I hope to get home at a reasonable hour tomorrow so I can check the resistance for the ground. I should know by then if the connections were soldered or not also. I have a long driveway, so I will just go back and forth while watching the fuel pressure until it dies. At that time I guess what I need to do is a max pressure test on the pump. Since the problem has been the same throughout, even when the pumps were new out of the box, I can't help but think the pump may not be the problem. If you (or anyone) can think of anymore advice, or address my other questions please do. Thanks again for all your help.

old_master
07-13-2009, 09:55 PM
....Key on engine off, pump running, I get about 62-63psi. When the pump stops, it drops to about 53-54psi and holds.

Close enough, that's fine.

I know when the pump was changed the SECOND time, the new plug/harness was installed. But I don't know if it was soldered or spliced using the supplied butt connectors. I'll find out. All I know is that he used heat shrink.

Very important, make sure they were soldered.

After reading your reply, I realized something. When the vehicle quit on me this weekend, I was by myself. So the only way I was getting pressure readings was turning on the key and walking around to look at the guage, which would read about 40-42psi. Since I now know a little more about how this system works, I can't help but wonder if the pump was supplying enough pressure, but the regulator was the culprit, not allowing enough to build up, or is that even possible? I guess the only way to find out is to drive it until it dies again. It's gonna be a hot week, so that should be easy.

Without isolating the pump, it's impossible to tell. Possibly a faulty check valve in the fuel pump.

I still don't understand how outside temp affects all this.

Probably coincidental.

What you said about the ignition switch has me intrigued. Can you give me some advice on what I should check? The only thing I know for sure is when it dies, I was getting the low pressure reading, and it would start when starting fluid (which I won't be using anymore) or gas is squirted into the throttle body. It will only run for a couple of seconds until that is burned, then back to square one. If it was a problem with the ignition switch, would there be codes? I don't remember if I mentioned it, but there has never been a SES light throughout all of this.

Fuel delivery is not monitored by OBDII so it can't turn the SES light on or set a DTC.

When the original fuel pump went out, it was a classic failure. She just went outside and the vehicle wouldn't start. She called me, and I had her depress the shrader valve after key on, no fuel pressure. She had it changed, all was well for about two weeks. Since then, this problem has been going on through two more fuel pumps. Changing the pump the second and then third time never changed the problem. At all.

Diagnosis will determine what causes them to fail prematurely. Be patient ;)



See if you can find out what brand of pumps were used. Keep us posted.

realdeal8471
07-13-2009, 10:00 PM
See if you can find out what brand of pumps were used. Keep us posted.

He said the last one put in was a Carter.

old_master
07-13-2009, 10:11 PM
Fuel pressure and leakdown rate are critical on these engines, they can be really fussy when it comes to fuel pumps. The most reliable ones are AC Delco and Delphi, that's all I'll install now. I've found that the least reliable are Airtex, Bosch and Carter. :frown:

realdeal8471
07-13-2009, 10:22 PM
I was told that the pump came in a AC Delco box. I have a friend that manages a NAPA store with 20+ years experience. He told me that a couple of different brands will be in a AC Delco box. Do you know if that is true? I had no idea. He said AC Delco doesn't actually manufacture the parts. I had the guy that did all of these pumps call the store and pull up the ticket, and it was definitely a Carter pump. If I have to change this thing, I will without a doubt go with Delphi. From reading this forum the last several days, that seems to be the brand everyone agrees on (even GM, obviously).

old_master
07-13-2009, 10:43 PM
I've never heard that about AC Delco pumps, sounds like false advertising to me. I have heard that Carquest pumps are now made by Delphi but they're in CQ boxes. I'm not certain, but I believe Delphi makes pumps and boxes them both AC Delco and Delphi. Maybe someone else here can verify/clarify this.

realdeal8471
07-13-2009, 10:49 PM
I agree with the false advertising if that is true. Also from what I have read of other people's nightmare problems, I think Airtex should be taken to criminal court. Starting to feel the same way about Carter. LOL.

realdeal8471
07-14-2009, 10:03 PM
Rain all day, didn't get to do anything. I guess my next step is to get it to stall again, then isolate the pump and do a pressure check. If that fails, I will go ahead and drop the tank and start checking the plug, harness, ohm the ground, etc. Thanks for your help, I will post an update as soon as I have one.

reevesval
07-28-2009, 04:00 PM
I have replaced the fuel pump 4 times in the last 6 months. Took it to one shop and they told me it was the fuse box under the hood had shorted out and was burning the pumps out. I replaced the fuse box and 6 weeks later another pump has burned out. I have checked the continuity on the fuel pump ground and get 1-4 ohms. Checked all the other connections on the pigtail and good continuity.

I have checked continuity from the fuse box and it is showing 2-5 ohms which says I have a dead short from the fuse box to the engine block. I have checked continuity across all the fuses (from leg to leg on same fuse) and I get high ohms on most as would be common across a light bulb. The only exception is the left and right rear parking lights which shows 1-5 ohms (a dead short)

My question, "could the ignition switch be defective and cause the short?" I disconnected the wiring harness from the bottom of the fuse box under the hood and checked continuity between the fuse bases and the main positive input to the fuse body and did not show a short. This tells me I have a short someplace in the wiring system.

Sometimes when traveling the car will suddenly die and all the lights will go completely out. I think this may be caused by the ignition switch. Anyone have any ideas? Your help will be very much appreciated!!!

Chris Stewart
07-29-2009, 07:44 AM
What year is yours?

reevesval
07-29-2009, 11:25 AM
My Bravada is a 1998 AWD V-6

Chris Stewart
07-29-2009, 07:59 PM
Man, I'm not good with wiring.
An electrician friend reminds me "low voltage= high current draw" when there's problems with burned up electric motors. Have you checked the voltage getting to the fuel pump?

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