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Truck Runs Real Rough, Please Help

doober

doober

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Location
River Falls, wisconsin
Hey Guys, My wife was driving our 95 6.5 suburban home from work this evening and said the truck started running rough all of a sudden at high way speeds. The oil pressure was normal and the water temp was normal so I told her to come on home.

She got home and I pulled the codes via. the paper clip method and this is what I found for codes,

Code 18 (Pump Cam Reference Pulse Error)
Code 19 (Crankshaft Position Reference Error)
Code 35 (Injection Pulse Width Error, (Response Time Short)
Code 54 (PCM Fuel Circuit Error)

I started the suburban in the drive way and it ram real rough and died in park when I tried to give it some fuel. I then started it back up right away and it ran okay and the service engine light went out. I shut it back off and started it right back up and it ran rough again and the service engine light came back on and I gave it alittle throttle and it even sounded like it was back firing just alittle bit and I shut it right off.

Anyone have any Ideas on what might be wrong? Really appreciate any and all the help you guys might have for me. Thanks.,
 
Thats real common when there is air in the fuel lines. You need to be real familiar with the lift pump (LP) operation and how to test it regularly, and drain water/sediment from the filter manager.

Rusty lines above the fuel tank, or a filter that wasnt properly seated, or if the LP is not functioning any little hole in the lines, corroded lines under the intake in the valley of death. Thats because IP resorts to sucking a vacuum and any little pin hole sucks air, compared to when pressurized.

LPs themselves commonly crap out, but so does the Oil Pressure Switch (OPS) that powers it when running. Which is why there is a OPS relay mod that is required for OBD1 trucks.

Check out the 6.5 technical library sticky for lift pumps. go through the whole things, as OBD1 trucks like yours is covered more in later posts.

So have you swapped a fuel filter recently, or put any additives in the fuel?
 
Guys, Thanks for the replys. My lift pump seems to be working. How I checked the lift pump I put the suburban in drive with out the engine running and then turned the key to start and could hear the lift pump run. Buddy, after reading your post I went back out and verified that the lift pump was running again and it was. This time I held the key on start longer to let the lift pump run longer and started the truck. I was hoping to make more pressure and then I started it. It ran alittle bit better this time. I didn't wan't to run it very long because the exhaust is loud and I don't want to wake the neighbors. It is possible that the ops is shot and is only operating the lift pump at the key start position? I will try to figure out how to jump the ops to get the lift pump to run constantly to see if that makes the truck run better when I drive down the road. Right now I can't even back out of the driveway because it is running so rough and it dies when I try to back out of the driveway. Heck it even sounds like it is backfiring alittle bit.

I have installed a new ac delco ops approx. 1 year ago. Not saying that it went bad again but it is fairly new. I think there was an orange wire along with two brown wires at the ops switch. I don't remember what wires I needed to jump to get it to run constantly though. I know I need to do the ops relay mod soon also.

Buddy, I will go through the library to look for the lift pumps. You might be on to something with the ops being bad already. The reason I say that is because when I put the truck in drive, before starting it, and then turn the key to start I hear the lift pump operate and pressurize the system. I then started it and it seemed to run better, or I think it did. Earlier Today I just started it without pressurizing the system and it ran real rough and even seemed to want to backfire. If anyone knows what two wires to jump at the ops that would be great. I will expermient on Tuesday alittle with that. I will open up the t-valve up by the thermostate and see if that pumps out fuel when it is running also.

It has been about 1 year and 7 thousand miles since I have changed the fuel filter. I run diesel service additive in every tank of fuel along with adding two stroke oil every tank also.

Veg Out, yes the codes do bother me also. I am afraid something might have gone bad in the pump? Well, Tomorow I will check the t-valve for fuel. Thanks for the good vibes and hope I start feeling them. I just get nervous when one of my trucks isn't running good. The codes that are coming up just make me worried that it might be something bad. But that is the way I am is that I always think the worst. Thanks you guys and if you can think of anything else that would be great. I will keep you posted on what I find out Tomorrow.
 
There is no way you messed up the IP's Optical Sensor (OS), the Crank Position Sensor (CPS), the IP fuel solenoid, and the PCM circuit and the thing still run at all.

These are common sucking air, fuel starvation codes.

Good trick with the putting it in gear and putting it in start is unique to the 95 year for running the LP. The ignition in 94 and ealier dont do that and the LP in later years doesnt work off the ign switch. However that doesnt test the OPS at all, just the LP and LP relay. start the truck if it will idle, and pop open the bleeder or drain to see if you get pressured fuel popping out or sucking that stalls the engine. Or when it stalls open it up and see if the filter bowl is dry, when it should be full.

And I would suggest a change of filter every 6 months even if you havent hit a mileage limit. It also needs to still be able to separate water effectively.

Of course another possibility is a snotty sock, also covered in the LP troubleshooting sticky.

OPS's die in a years time easy, which is why the relay mod is so important. For OBD1 trucks The relay mod probably keeps the truck on the road and no worried phone calls, about as much as the remote PMD mod does.

The OPS orange and grey wires are for the lift pump. The tan/brown goes to the oil pressure gauge.
 
If you do get fuel flow at the t-valve drain and the truck doesn't die when you open the valve, you've got lift pump operation. If no flow or the truck dies, lift pump isn't operating. Keep in mind that a lift pump you hear clicking away steadily still may be shot, not providing pressure.

A fuel pressure gauge is a great help, you can hook it up to the t-valve drain and see if you have any fuel pressure, assuming the above test shows your lift pump is operating.

1st change fuel filter, check for crud in the filter manager. 2nd prove the lift pump is running (meaning OPS is OK). 3rd prove lift pump is providing volume and pressure (fuel flow at the t-valve, pressure on the gauge). If all this proves out, follow Buddy's suggestions about looking for air leaks or tank sock problems. Hope this helps!
 
Your best bet if you have fuel pressure is to sight glass it. The tiniest bit of air in the fuel will cause it to run horrible if at all. The leak will be pre LP if you have one. Very common for the Pickup to rot right at the bend where it goes into the fuel tank. Verify fuel pressure then sight glass the supply line to the IP. ANY bubbles at all and that is your problem.
 
Guys,Thanks for all your great ideas. What I did this morning is I took the fuel cap off. There was air that whooshed into the tank which I believe is normal. I then just screwed the fuel cap on 1/4 turn so I don't create a vacume in the tank for now. I then got an empty clear container and put the t-valve hose in it. Then I got in the truck and put it in drive and turned the key to start and let the lift pump run for about 1/2 minute. Then I started the truck and it ran like a million bucks. I got out and turned the t-valve open and watched the good floe of fuel start filling the empty jug. The engine didn't miss a beat with the valve open so I closed it. I went to town and got a cup of coffee and came back home. Ran perfect.

I could swap fuel caps with my 92 6.5 to get ride of the possible fuel cap problem. It could still be the ops I guess. I suppose it could also be a weak lift pump? It's hard to trouble shoot something when it is working good as I think everything is working good now. It might be a different story if I drive it for a few miles though.

Does this intermintent problem sound familiar to anyone? Thanks for all your help guys.
 
You may have solved it. No, the whoosh is not normal, your cap needs to vent to not build vacuum in the tank. Swap the caps and cross your fingers, could be the cheapest fix of the day!
 
Sounds good Dave, I was under the impression that there should be some vacume in the tank. That negative pressure is there so the fuel doesn't foam up and then the foamed fuel would get sucked up and end up at the injection pump causing running problems?
 
There is supposed to be vacuum and that is normal, the cap only vents to maintain a specific negative pressure and not too much. If people dont get that then there is a hole in their fill tube or tank.

Air leaks can be intermittent, and OPS operation can be intermittent. I had a new OPS that after about 8 months it would drop from battery voltage to 7V and the fuel pressure would go to vacuum. turn off truck and restart and it came back up to 14v. You can see the LP voltage from the OPS on pin G or F on the OBD connector when the truck is running. bottom or top left contact.

Can always be the snotty tank sock, especially if you were low on fuel.

Can still be a pesky air leak above the fuel tank. Popping off the vaccum of the fuel tank has temporarily "helped" other people with rotted pickup fuel lines.
 
Sounds good buddy, So your saying that if I took a voltage reading on that connector under my dash, where the code reader would plug in, I would be able to get readings from that area? I could take my meter and place one probe on a ground source and the other probe on pin (g) or (f)? And those two pins are the ones on the far left of the connector? Thanks.
 
There is supposed to be vacuum and that is normal, the cap only vents to maintain a specific negative pressure and not too much. If people dont get that then there is a hole in their fill tube or tank.
Click to expand...
Buddy, after sitting all night you'd expect to get a whoosh when you remove the fuel cap? I understand a little is OK, but these engines will run fine with no vacuum, won't run with too much vacuum, and once he vented his truck is running fine.

I realize it could be an intermittant OPS or other problem, which we'll see over the next day or so as he runs it, but seems like that fuel cap might have been the cause.
 
doober said:
Sounds good buddy, So your saying that if I took a voltage reading on that connector under my dash, where the code reader would plug in, I would be able to get readings from that area? I could take my meter and place one probe on a ground source and the other probe on pin (g) or (f)? And those two pins are the ones on the far left of the connector? Thanks.
Click to expand...

Yes exactly correct. The normlly closed contact on the LP relay goes back to the OBD connector, and so when the LP relay is not energized by the ignition in Start, the power from the OPS going to the LP sits on that normally closed pin of the LP relay, which in turn is connected to the OBD connector. On 94s it is pin G, and Ive seen schematic that moved it to pin F in 95s. It allows scanners/technicians to check it. They are the two left most pins. You can also power the LP by putting 12V to that OBD connector contact.
 
Dave01 said:
Buddy, after sitting all night you'd expect to get a whoosh when you remove the fuel cap? I understand a little is OK, but these engines will run fine with no vacuum, won't run with too much vacuum, and once he vented his truck is running fine.
Click to expand...

Yes, it is a closed system and one in good order will hold the negative pressure. The venting cap does not simply vent, it holds the negative pressure to a specific range. If it goes more negative it vents and then closes again as soon as pressure is the correct range. The more fuel that you have used since opening it, the more whoosh there will be, because the more volume of vacuum.

The absence of vacuum can temporarily help alleviate leaks because it may become easier for the LP to pull and return higher volumes of fuel with no vacuum, and less potential for air to seap in a pin sized hole. does not solve the root cause that the fuel lines between the tank and LP are probably rotted and allowing air to enter, so will happen again on a random on ramp to the interstate when trying to accelerate.
 
Thanks Buddy, I was just researching the f and g pins. Do you know where I might find power under the dash near or at the DLC connector? I am thinking of making a jumper wire to the f pin on my 95. I would need 12v power and connect it to the f pin correct? That would make the lift pump run all the time, for testing ops function?
 
Sorry for being brain dead, but I just want to make sure I get it right so I don't create a dead short and burn something out here. Buddy, I reread your posts and it looks like I could put a 12v power source to pin f and that would run the lift pump? If that is true would you know where a 12v power source is in that area or would I have to run a wire up to the battery? Just trying to trouble shoot this in a way I understand it best. Thanks.
 
You can run the LP by putting a 12V source to pin F on the diagnostic conenctor. you do not want to ground it. While running you should also be able to see 14V there coming from the OPS. Its ok to have a parallel power source so you wont hurt it by adding another 12V source.
 
Okay just got back from doing some testing. I put a meter on the DLC connector on pin f and grounded the other side to check for voltage at the lift pump. All this was hooked up so I could drive around and watch for a voltage drop at the lift pump. It stayed at a constant 12 volts no mater what speed or how hard I pushed the throttle. It would run okay until I got on the throttle and then it would start to miss and run rough and the service engine light would come on.

So lift pump appears to be running all the time if pin f to ground is the right way to test the lift pump operation. I then borrowed a gauage and hooked a hose up to the t-valve in front of the engine. I ran the hose under the hood and into the window. Took it for another test drive. I was getting 4 1/2 on the pressure guage at idle. When I gave it throttle, enough to make it start spitting and running rough I was down to 2 1/2 on the guage. It would start missing as soon as I gave it hard throttle though before it even went down in pressure. I am not sure how accurate his gauge is but these numbers seem okay, don't they? As I let off the throttle and coasted the pressures went back to the 4 1/2.

I have a new filter laying around here I think. I wonder if I should install it? Darn, it looks like the ops is working and the lift pump is also working okay?
 
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