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97 GMC C1500 P0175 and P0137: Hard Start, Rich Running, and Fuel Leak Diagnosis

Model: 97 GMC C1500 Fault Code: P0137 Posted: 2007-04-04 12:23

I recently experienced a hard start on my 1997 GMC C1500 after the engine had been sitting for several hours. When I restarted it immediately, there was no misfire or stumble—this indicated that a fuel-related issue was triggered by idle time. The symptoms pointed to either a fuel leak or insufficient fuel delivery. I began by checking fuel pressure and volume, which showed normal readings under operation. However, I observed visible bleed-down on the fuel pressure gauge during idle. This bleed-down did not cause a hard start when restarting immediately, as the engine started smoothly without any hesitation or misfire. After removing the upper intake manifold, I noticed a small amount of fuel dripping from the vacuum regulator—though minimal, it was significant given the vehicle's poor performance. I then reconnected the entire fuel system with the upper intake removed and turned the ignition on (engine off). No visible fuel flow occurred under these conditions. However, at idle, vacuum pulls fuel through the regulator and atomizes it, so a small leak may not be immediately visible. After replacing the complete fuel injection unit—now including individual injectors—the hard start issue was fully resolved. Note: The bleed-down persists but does not appear to affect engine performance under normal operation. While some bleed-down is expected in older systems, excessive or uncontrolled leakage can indicate a failing fuel pump or seal. A check ball inside the pump typically prevents such bleed-down. An O-ring failure may allow air ingress and cause lean conditions if fuel levels are low. Additionally, I observed significant soot coming from the exhaust, with intermittent spurts that smell like gasoline—likely due to unburned fuel entering the exhaust system. This strongly suggests a fuel leak somewhere in the intake or injection system. When the upper intake was removed, the interior appeared sooty and dry, indicating incomplete combustion. To further diagnose, I recommend removing the upper intake, pulling out the injector tubes (by releasing clips on each side), and disconnecting the central injector controller to allow better inspection. With the ignition ON but engine OFF, observe for fuel leakage—this will likely reveal a leaking injector or tube. The newer fuel injection unit relocates injectors to the cylinder ends, which improves durability and reduces risk of breakage during installation. In summary: The P0175 (System Too Rich Bank 2) and P0137 (O2 Sensor Circuit Malfunction Bank 2) codes are consistent with a fuel delivery issue. While the fuel pump may be functioning, a leaking injector or tube is likely the root cause. Replacing the updated injection unit resolved the hard start and rich running symptoms.

Related fault codes

P0137P0175

Comments (8)

Anonymous 2007-04-04 12:30

GM Parts Direct has the replacement part available for $230—currently on its way. Could a loose O-ring on the fuel pump cause bleed-down? While some bleed-down is normal, does it mean the pump has to rebuild pressure every time the engine starts? I'm planning to test my fuel pressure gauge on my Tahoe tonight; since it runs well, the reading should be reliable. Thanks, Chad

Anonymous 2007-04-04 14:00

A small amount of bleed-down is not uncommon in GM vehicles and may not cause issues if the engine starts smoothly, builds pressure properly, and operates without whining. In my experience with a daily driver, I left the pump as-is and haven’t had any problems. Most fuel pumps have a check ball that prevents bleed-down—this helps improve cold start performance. A leaking O-ring could allow air into the system (especially if fuel level is low), causing lean conditions. Bleed-down doesn't necessarily mean the pump is faulty; it's often a matter of trade-offs. If your truck runs fine, saving money and living with minor bleed-down may be acceptable.

Anonymous 2007-04-04 14:17

I also noticed heavy soot coming from the exhaust, now with intermittent spurts that smell like gasoline—likely due to fuel entering the exhaust stream. My phone camera is poor, but I can see a lighter-colored puddle in the middle of black soot, which appears to be liquid leaking out. This is especially concerning since the exhaust was recently replaced. What do you think this indicates? Thanks, Chad

Anonymous 2007-04-04 14:35

This clearly points to a rich fuel condition. When you removed the upper intake, what did it look like inside—wet and clean or sooty and dry? If there's significant soot in the exhaust, there’s likely a major fuel leak somewhere. Next time you remove the upper intake, inspect under it for wet spots indicating fuel leakage. Since your vacuum regulator was replaced, an injector or tube is probably leaking badly. You don’t have a fuel delivery issue—your pump is working correctly. Focus on identifying the source of the leak. To check: disconnect the injector tubes and raise the central injector controller (using the bottom screw), reconnect the fuel lines, turn ignition ON, let the pump pressurize, then look for leaks. I bet you’ll see evidence of a leaking injector. Also note: newer units relocate injectors to cylinder ends—this design prevents tube breakage when bent during installation. Just follow instructions carefully and don’t force them.

Anonymous 2007-04-04 17:42

Here’s a photo of my intake—I know it's dirty, but my phone camera is poor. My mechanic used engine cleaner through the brake booster hose, which only reached the rear four cylinders; the front remains much dirtier—likely due to an EGR valve in that area. The two small tabs on each injector are very fragile. When I tried reinstalling the intake, I wiped down the mounting surface and accidentally snapped off two of these tabs with a paper towel. I don’t think pinching them will work. Can I just pull them out or do they clip into place? I’ll have to wait at least a week for new parts to arrive—so I’m working on other fixes in the meantime. I’ve ordered the newer model with individual injectors at each cylinder—hopefully this resolves the issue. Thanks, Chad

Anonymous 2007-04-05 14:30

I recall breaking a couple of those clips too. What I did was remove the broken pieces to prevent them from falling into the engine bay. Then, using small pen-sized screwdrivers, I gently squeezed in slightly where the clips were broken while prying outward with the rounded edges of the intake ports as leverage. I used rags or heavy-duty paper towels to cover the intake runners during this process. Be very careful—don’t break the tubes or let any pieces fall into the cylinders. Try pulling and wiggling the clips at the same time. A little penetrating oil might help loosen them (just a small amount—no liquid in the cylinder). This advice isn't ideal, but it’s one of those practical fixes. If you try it, plug everything back in and inspect for leaks. I’ll check the forum again tonight for updates.

Anonymous 2007-04-05 14:57

I won’t be able to open the engine again for a while—waiting for the spider injector to arrive. Your advice was incredibly helpful. I’ve saved this site as a favorite because of the reliable information here. I’m now doing more hands-on work on my truck myself, and it’s already saved me significant money compared to going to a mechanic. Thanks so much for your support.

Anonymous 2007-04-06 09:13

No problem—happy to help!