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b_oneself
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Dec 29, 2011, 4:11 PM
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O. K.,
I have a 98 GMC Savannah 3500 van (box truck) with a 5.7 engine. It starts fine, every time and has plenty of power. However, after it gets plenty warm (a least an hour or two of driving) there is a huge loss of power. At that point the engine light comes on (OBD II codes P0131 and P0151) and it feels like it is running on 2 or 3 cylinders -it really bogs down when you accelerate even though its continues to idle just fine. Here is a long winded explanation of what I have done thus far: 1) A mechanic friend hooked up a vacuum gauge and was convinced it was indicating some blockage in exhaust route so the two catalytic converters were replaced and the exhaust pipe inspected (all good) except for the section from the front section up to the manifold which I guess couldn't be seen. Next we went after the two upstream O2 sensors and replaced them. After that we replaced the dist. module, the coil and the pick-up inside the dist. The fuel pump is putting out 45-52 psi when the engine is cold (full power) and with the fuel pressure gauge attached while driving remains steady even when the power problem begins. In other words, no difference in fuel pressure during the malfunction. I don't know where to go from here. P0131 = O2 sensor circuit low voltage(bank 1 sensor 1) P0151 = O2 sensor circuit low voltage (bank 2 sensor 1) Can anyone out there give me a clue about what to do next? Thanks for being there. Bob
(This post was edited by b_oneself on Jan 1, 2012, 7:37 PM)
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b_oneself
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Dec 29, 2011, 4:35 PM
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Re: Power problem
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Thanks for such a quick reply. But if you say it needs a minimum of 55psi (I also heard 60- 66psi), why does it run so well at the beginning? Is something going on with the FI system that when the engine gets hot 47 psi is insufficient whereas being adequate enough for the first hour or two of driving? Bob
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Hammer Time
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Dec 29, 2011, 4:44 PM
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A lot depends on temperature and engine load but I can tell you positively that a Vortec will be nothing but problems under 55PSI. Yes it should be higher but 55 seems to be the cut off point.
Here is the actual spec
Pressure 55-60 psi Open Throttle 66 psi
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b_oneself
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Dec 29, 2011, 7:13 PM
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Re: Power problem
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Hammer time,
Thanks again for your attention to my problem. I'm going to take your advice and get a new fuel pump (inside gas tank). One of the reasons I was reluctant to change to fuel pump was that I already previously replaced it a few years ago (maybe just 2 years ago) and found it hard to believe it has already gone bad. The vehicle does put a lot of miles on it per year. (about 200 K on the engine rebuild so far). Also I mis-spoke about the trouble codes. They are actually P0131 and P0151 (O2 sensor low voltage bank 2 sensor 1) One last thought. Is there possibly a fuel pressure regulator that is accessible and adjustable? Any way, I'll report back after installing the new pump - hopefully tomorrow.
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Hammer Time
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Dec 29, 2011, 7:20 PM
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I didn't advise you to change the fuel pump. I advised you to find out why the fuel pressure is too low. Don't jump to conclusions before verifying. Find a place to pinch off the return line and see if the pump will deadhead to higher pressure. If not, verify that there is no voltage drop though either the power feed or the ground wire at the pump connector. They are known to have a lot of problems with the connectors too. If you do change the pump, beware of the Chinese junk that seem like great deals. Delco is the only brand I will recommend. Carter is OK, but they still have some problems.
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b_oneself
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Dec 29, 2011, 8:38 PM
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I stand corrected. So should I assume that if the fuel pressure does not deadhead to higher pressure -and it is not because of a voltage drop (13.2 constant volts?) that the fuel pump or pressure regulator is bad? And where is the pressure regulator in my engine? Also, I failed to mention that this pump screams with a loud high whine. I'll conduct the tests you suggested and check back. Thanks for the tip about Delco. For all your help -much appreciated!
(This post was edited by b_oneself on Dec 29, 2011, 8:44 PM)
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Hammer Time
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Dec 30, 2011, 4:00 AM
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No, when you test the dead head pressure, you are testing the regulator. If the pressure does spike up, that's when you look at the regulator which is under the intake manifold. Voltage and regulator eliminated, then blame the pump or internal tank problem.
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b_oneself
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Dec 30, 2011, 6:13 PM
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Master,
I followed all your instructions, I hope correctly, and here are the results. The fuel pressure does spike significantly (it surged 20 psi during three separate tests) when I compressed the return fuel line. I simultaneously recorded the voltage and this is where it gets a little nebulous to me (being under-educated in the finer points of electricity). To begin with I measured the voltage at the battery with the engine running and got 14.3 volts but when I check the power feed and ground to the fuel pump I get about 12.5 (it varies -up to 12.7). Furthermore, I finally realized that the last time I changed the fuel pump, I must have spliced into the existing harness to install an alternative replacement pigtail that came with the pump. After remembering this and I figuring that my crimping connections might be faulty I rechecked the voltage at the power feed and ground at the very end of the harness (after my splicing area) ) and got less voltage -12.2 In both instances, before and after my crimped connections, there was a voltage drop of about .5 volts when I tested the dead head pressure. Still, from how you explained it, I gather that the pressure regulator (in the intake manifold) needs looking into. Am I on the right track?
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Hammer Time
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Dec 30, 2011, 6:22 PM
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Yes, that seems to be what your tests point to. If you can spike the pressure that high with pinching off the return, it pretty much tells you that the pump is capable of doing what it's supposed to and the regulator seems to be bypassing at too low of a pressure. Just make sure your numbers are correct. You probably don't have a voltage issue but I'll give you another way to test for voltage drop. You're going to need a long, reliable jumper wire. Tap into that gray feed wire as clost to the tank as possible without disconnecting anything. Run the wire up toward the engine and dig out your voltmeter. Attach one lead of the voltmeter to the jumper wirte and the other lead to the positive battery terminal. NNow energize the fuel pump somehow and watch the meter. It will measure the amount of current that you are losing. Anything over half a volt would be too much.
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b_oneself
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Dec 31, 2011, 2:09 PM
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OK, so I ran the voltage drop test according to your instructions. The method I used to energize the fuel pump was a) starting the car, and b) turning the key to the On position where the pump briefly pre-loads . The results are: for a) 1.7 volts and b) 1.6 volts Note 1 -my tapping in spot into the gray power feed to the fuel pump was after my previous splice area about four inches from the pump plug-in. Note 2 -at the beginning of this whole investigation I added an additional main power feed (cable) from the positive post to the central power junction box with the fuel pump relay at the driver's corner (opposite the battery) because I had noticed the original wire was extremely corroded. I did not remove the original one because I was afraid I'd break the fragile plastic connector terminal. Instead I just bolted up to the auxiliary ? post right next to it. Note 3-when I squeezed the return fuel line to dead head the pressure, which succeeded (pressure surged 20psi), at the same time the RPMs dropped significantly and also seemed to run smoother.
I am wondering if I need to address these three issues ...the automotive connectors I used at my spice-in point to the fuel pump, the apparent voltage drop to the pump, and the original power problem somehow related to the fuel pressure regulator in the manifold?
(This post was edited by b_oneself on Jan 1, 2012, 7:43 PM)
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Hammer Time
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Dec 31, 2011, 2:23 PM
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You have determined that you do have a considerable voltage drop that has to be address before doing anything else. You now need to find out where this problem is occurring. You do this by repeating that same test at various points in the circuit until you find the point where it isn't there any more.
I would start with the fuse itself and then both in and out of the relay.
Here is the wiring diagram if you don't already have one.
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b_oneself
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Dec 31, 2011, 2:34 PM
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Much obliged. I will look into the voltage drop first. Thanks again for the all the support -the wiring diagram, etc. This forum is so educational to us amateurs and you, Hammer Time, in particular, are a great teacher.
(This post was edited by b_oneself on Jan 6, 2012, 1:47 PM)
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Hammer Time
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Dec 31, 2011, 2:35 PM
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Thank you
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b_oneself
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Jan 1, 2012, 7:04 PM
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Okay,
After numerous and repeated tests concerning the voltage drop to the fuel pump, I have isolated the problem as occurring after the fuse box/fuel pump relay center (eliminating both fuse and relay I presume). Both the grey power lead wire, and the black/white ground wire coming out of the fuse box have virtually no voltage drop. From that point on (they pass through one factory junction/plug and then through my splice job), a total of between 1.2 and 1.6 volts gets lost- probably 20 feet to the the pump. The hot wire loses approximately 1.0 volts, the ground .5 volts. Regardless of this proven deficiency I find it hard to believe this is the cause of my fuel pressure problem (min. 55psi). Here's why: I temporarily removed the relay -(to be safe). Then I ran a long thick wire from the positive post on the battery the entire length to the fuel pump at the fuel tank. I gained .9 to 1.1 volts. Then I grounded the ground wire at the fuel pump and gained another approx. .5 volts. During each of these voltage boosts there was no change whatsoever in fuel pressure. I recovered approx. 1.2-1.5 volts using my thick testing wires (bypassing the relay and reading 13.6 -13.9 volts at the fuel pump with the engine/alternator running. There is plenty of volts at this point with no more than .5 voltage drop) Still, no change in fuel pressure -it does what it does every time -jumps to 60 psi as soon as the key is on then hovers around 52 psi (with tiny fast fluctuations) once I start the engine and finally sometimes dropping to as low as 47 psi. As soon as I turn the key off it jumps right back to 60 psi and then sometimes slowly eases back to 47 psi.
I've got to go back to your axiom : "if voltage and pressure regulator check out then you suspect the pump". Well the voltage checks out now, (temporarily), but there is still the issue of pressure spiking when the return line is pinched -(the fuel pressure regulator inside the manifold -right?) So aside from the voltage drop problem, I've still have to deal with the regulator. Is this a major amount of work?, an adjustable unit?, expensive? Have I missed something in my analysis, or forgetting something in my (your) diagnostic process? Sorry to keep bugging you. Happy New Year!
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Hammer Time
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Jan 1, 2012, 7:12 PM
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No, the regulator is not adjustable and just has to be replaced. It's not a real big deal. You just have to remove the upper intake.
It would be a good idea to resolve the voltage drop problem regardless because it will lead to early fuel pump failure if left that way. It probably a bad connector somewhere.
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b_oneself
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Jan 3, 2012, 11:12 AM
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Hammer Time,
Thanks to your guidance, I was finally able to remove the upper intake manifold in order to replace the fuel pressure regulator. I noticed the entire cavity was pretty dirty (oily sludge) especially around the piece the regulator plugs into. I was just wondering how important or necessary is it to clean this area (how clean?) and is this gunk indicative or related to my fuel pressure issue? I hope to install the new regulator today and then re-test the fuel pressure.
(This post was edited by b_oneself on Jan 3, 2012, 6:42 PM)
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Hammer Time
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Jan 3, 2012, 11:37 AM
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Don't worry about that. It's a good sign. Clean spots means leaking fuel.
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b_oneself
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Jan 3, 2012, 3:57 PM
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Okay,
I accomplished the fuel pressure regulator installation (not that big a deal, I guess, once one's familiar with the procedure) but RATS I've got the same exact fuel pressure readings, the same 20 psi spike when I dead head the fuel pressure, and I know I'm getting full voltage (13.9-14.3 volts at the fuel pump) because of my temporary supplemental wires. What next? Shouldn't the new pressure regulator have at least solved the surging pressure issue? One more possibly related note. During some of my pressure testing readings (with the engine running) I noticed the plug to the distributor was disconnected. I was surprised to see it since I had just replaced the pick-up and figured it must have gotten knocked off when I took the engine cover off. I was completely perplexed why the engine could be running at all without it being connected. When I mentioned this to my mechanic friend he concluded the engine must have a crankshaft sensor that enables fuel to be dispersed. He also commented that his description of the basic problem is that "it sounds like the engine is running up its rev limits". Anyway, I feel stuck again, a little poorer, but still determined to get to the bottom of this.
(This post was edited by b_oneself on Jan 3, 2012, 3:58 PM)
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Hammer Time
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Jan 3, 2012, 4:01 PM
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You need to try a different pressure gauge. This doesn't add up at all. We could be chasing a ghost if your gauge is wrong.
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b_oneself
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Jan 3, 2012, 6:33 PM
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Mr. Wizard,
No, no ghosts here, but you sure did sniff out the culprit in the mystery. After cross-testing with another fuel pressure tool, I get completely different readings of fuel pressure (and it was a top-quality Snap-on tool...who would've thunk it). Okay, so here are the present readings: 67 psi -key on, 57 psi -engine running - everything holding steady, even with no extra added voltage. So, given the new information, where do I proceed from here (back to the codes?) I haven't done the on the road fuel pressure test, yet, under the very warm (an hour or more later) condition that consistently precedes the power problem the engine is having. Thanks again for steering me in the right direction when things can get so frustrating.
(This post was edited by b_oneself on Jan 3, 2012, 6:37 PM)
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Hammer Time
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Jan 3, 2012, 6:43 PM
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Well, that solves one question but puts us right back to square one.
I guess the road test with the fuel pressure gauge attached would be a good idea.
I think we need to revisit the restricted exhaust theory, especially if you are getting a low vacuum reading. There is an easy way to test this using a pressure gauge threaded into the 0/2 sensor hole.
I don't know if you have access to a good scan tool but it woud be nice to see a data snapshot as this is occurring.
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(This post was edited by Hammer Time on Jan 3, 2012, 6:46 PM)
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Discretesignals
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Jan 3, 2012, 6:49 PM
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If the exhaust was restricted and making the engine lose power, why does it run fine until you have driven for a period of time?
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Jan 3, 2012, 6:50 PM)
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Hammer Time
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Jan 3, 2012, 6:51 PM
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I've seen many cats act that way.
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Discretesignals
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Jan 3, 2012, 6:58 PM
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I was just curious because he was checking fuel pressure when the engine was running fine, I assume. He also had two low voltage codes at the O2 sensors, which could mean it was leaning out from lack of fuel. Maybe the pump is getting hot and slowing down causing a loss of fuel pressure/volume.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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