I got Car Code for my '99 6.5TD. (www.obd-2.com)
I was planning to install all kind of gauges, but when I looked at the wiring diagram it seemed that GM already built sensors into the engine for nearly every gauge I wanted. Rather than buy and install lots of redundant sensors, and fuss around with how and where to mount all these gauges, I plug a little netbook into the OBD port and display whatever I want on my soft dashboard.
The only sensor GM didn't include is fuel pressure between the filter and IP. I have a mechanical gauge there, reading 30HG to 10PSI. I'll upgrade to an electrical gauge eventually, but the mechanical gauge serves my needs for now. It's currently reading pressure on the dirty side of the filter. I'll change it to the clean side when I do FTB.
Now that I can see data from all the sensors in the engine, I have a million questions. These generally fall into three categories -- where is the sensor, what readings are normal/marginal/bad, and what do I do about marginal/bad readings?
For example, OBD reports fuel temperature. Where is the sensor? I'm guessing this is fuel temperature within the IP, but that's just a guess. It also shows high and low fuel pressures, which I assume are within and after the IP, but again I'm just guessing. The units on these sensors seem wrong for the pre-IP pressure I want to see.
Can anyone refer me to a good source of this kind of information? I've searched the technical documents on this site. I've googled. I've gone through our town library system, including its on-line auto repair data repository.
As an example of the information I seek, below I've written up my understanding of the pre-IP fuel pressure gauge I've installed. I think I understand this sensor pretty well. I would like to have this level of understanding of all the sensors the 6.5TD reports via OBD-II with its extensions, particularly things like boost and EGT.
Thanks,
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Here is my understanding of pre-IP fuel pressure on the 6.5TD.
The IP is designed to have 3-5PSI on the input side. If the pressure drops below 3PSI, we have a problem. This can happen due to a dead LP, bad OPS, clogged fuel filter, clogged tank sock, fuel line leak, seal leak, etc. I know how to jump the fuel relay to bypass it and the OPS. I know how to test fuel pressure on the clean (death valley) and dirty (bleed hose) sides of the filter, to detect a clogged filter. I know how to blow low-pressure air through the bleed hose to temporarily clear a clogged sock, and that I need to clean the tank if that's my problem.
I understand the impact of low fuel pressure to the IP. Mildly low pressure just makes the IP work harder, increasing wear. Negative pressure (vacuum) can cause dissolved gasses to become bubbles. Vacuum can also cause seals designed for pressure to allow air into the line. Bubbles don't lubricate, increasing wear on the pump. At high pressure, bubbles can abrade metal.
If the LP dies, the IP sucks fuel in anyway, and pre-IP fuel pressure goes negative. I've seen over 10HG on the dirty side of the filter. When my LP died, it continued to make the usual sound, OBD reported it continued to draw the usual amount of power, and the engine ran fine. But my IP was taking a beating. Fortunately I discovered the problem within 10 miles, because I was frequently bleeding to check for water in my fuel.
Too-high pressure to the IP causes a different set of problems. There is a fuel return line that runs from the IP to the tank. That line has a pressure valve that cracks open at 5PSI, sending excess fuel back to the tank as needed to keep the pressure from exceeding 5 PSI. If that valve sticks open, the LP can struggle to maintain 5 PSI at the IP. If that valve sticks closed, pressure to the IP can climb to the point where seals start to fail.
So a pre-IP fuel pressure gauge should ideally read 5 PSI at all times. FTB plus a strong lift pump can achieve this. Dropping to 3PSI does no harm, but is not ideal. Pressures outside of 3-5 PSI are outside the design parameters of the IP.
I was planning to install all kind of gauges, but when I looked at the wiring diagram it seemed that GM already built sensors into the engine for nearly every gauge I wanted. Rather than buy and install lots of redundant sensors, and fuss around with how and where to mount all these gauges, I plug a little netbook into the OBD port and display whatever I want on my soft dashboard.
The only sensor GM didn't include is fuel pressure between the filter and IP. I have a mechanical gauge there, reading 30HG to 10PSI. I'll upgrade to an electrical gauge eventually, but the mechanical gauge serves my needs for now. It's currently reading pressure on the dirty side of the filter. I'll change it to the clean side when I do FTB.
Now that I can see data from all the sensors in the engine, I have a million questions. These generally fall into three categories -- where is the sensor, what readings are normal/marginal/bad, and what do I do about marginal/bad readings?
For example, OBD reports fuel temperature. Where is the sensor? I'm guessing this is fuel temperature within the IP, but that's just a guess. It also shows high and low fuel pressures, which I assume are within and after the IP, but again I'm just guessing. The units on these sensors seem wrong for the pre-IP pressure I want to see.
Can anyone refer me to a good source of this kind of information? I've searched the technical documents on this site. I've googled. I've gone through our town library system, including its on-line auto repair data repository.
As an example of the information I seek, below I've written up my understanding of the pre-IP fuel pressure gauge I've installed. I think I understand this sensor pretty well. I would like to have this level of understanding of all the sensors the 6.5TD reports via OBD-II with its extensions, particularly things like boost and EGT.
Thanks,
-jpg
Here is my understanding of pre-IP fuel pressure on the 6.5TD.
The IP is designed to have 3-5PSI on the input side. If the pressure drops below 3PSI, we have a problem. This can happen due to a dead LP, bad OPS, clogged fuel filter, clogged tank sock, fuel line leak, seal leak, etc. I know how to jump the fuel relay to bypass it and the OPS. I know how to test fuel pressure on the clean (death valley) and dirty (bleed hose) sides of the filter, to detect a clogged filter. I know how to blow low-pressure air through the bleed hose to temporarily clear a clogged sock, and that I need to clean the tank if that's my problem.
I understand the impact of low fuel pressure to the IP. Mildly low pressure just makes the IP work harder, increasing wear. Negative pressure (vacuum) can cause dissolved gasses to become bubbles. Vacuum can also cause seals designed for pressure to allow air into the line. Bubbles don't lubricate, increasing wear on the pump. At high pressure, bubbles can abrade metal.
If the LP dies, the IP sucks fuel in anyway, and pre-IP fuel pressure goes negative. I've seen over 10HG on the dirty side of the filter. When my LP died, it continued to make the usual sound, OBD reported it continued to draw the usual amount of power, and the engine ran fine. But my IP was taking a beating. Fortunately I discovered the problem within 10 miles, because I was frequently bleeding to check for water in my fuel.
Too-high pressure to the IP causes a different set of problems. There is a fuel return line that runs from the IP to the tank. That line has a pressure valve that cracks open at 5PSI, sending excess fuel back to the tank as needed to keep the pressure from exceeding 5 PSI. If that valve sticks open, the LP can struggle to maintain 5 PSI at the IP. If that valve sticks closed, pressure to the IP can climb to the point where seals start to fail.
So a pre-IP fuel pressure gauge should ideally read 5 PSI at all times. FTB plus a strong lift pump can achieve this. Dropping to 3PSI does no harm, but is not ideal. Pressures outside of 3-5 PSI are outside the design parameters of the IP.
