Hello all, I'm new to the forum and hoping someone may have some insight for me. I'm going to be as thorough as I can so I apologize if I'm long winded. I'm not a certified mechanic, but have been around cars my entire life and have done all of my own mechanic work for many years. I have a very well maintained 96 Tahoe LT Z71 with a 5.7 vortec with 70k pampered miles. The truck is garaged, in perfect condition and basically a weekend driver. I'm the 2nd owner since 2001. A few months ago I started experiencing hard starting and a very slight fluttering low rpm hesitation on the highway at around 70 mph at around 2000 rpms. The hard starting got worse until it wouldn't start at all. I did fuel pressure tests and decided to replace the fuel pump. Dropped the tank in the driveway, that's always fun. Installed a new AC Delco full assembly pump and sending unit. Truck fired right up. But after getting it on the road, still had the highway hesitation. After a few outings, the SES light came on. Actually the light had been on and off prior to replacing the pump, but I didn't have a scanner at the time. So now I scanned it and got a P0300 multiple misfire code. The truck idles perfectly and runs fine. So just to be certain I immediately went for the obvious ignition parts first and installed all new AC Delco cap, rotor, plugs, and wires, cleared the code, drove it for a while and still the same hesitation. SES light came on again and it's now P0308 misfire cylinder 8. I have read many forums saying that the spider injectors or pressure regulator could be the culprit. I also am aware of the notorious leaking intakes on this model. About 10 years ago I fell victim to the leaking intake gasket and had it replaced. Since my new symptoms I've checked the intake and there aren't any leaks or coolant in the oil or in the oil filler cap. I sprayed the entire thing with carb cleaner and can't find any vacuum leaks. I also read the distributor rectifier (I believe that was the name) is also a common problem on these models. I checked fuel pressure and have about 58-60 psi key on not running, 52 running, and jumps over 60 when hammering the throttle, and maintains 54-56 while driving. Everywhere I've read 60 - 66 psi is optimum. So I'm leaning toward spider and regulator, but am not certain as I clamped off the fuel return line and key on not running It read 74 psi. After 5 minutes it only dropped to 64 psi. I'm thinking a leaking injector or regulator would drop pressure much more than that. So maybe very slight leak and/or clogged injector creating misfire? I'm aware that a bad coil, control module and any one of the multiple sensors could cause this problem, but I would expect a different code and I'm still leaning towards fuel related parts. I would opt for the spider injector csfi to mpfi upgrade kit as well including new regulator. I haven't done a compression test, but I refuse to believe such a well maintained low miles vehicle could have a bad valve or rings as the idle is really smooth and it has plenty of power. Any help would be greatly appreciated. I just want to make sure I'm on the right track, and really want to get my Tahoe back in shape. Thanks in advance !
96 tahoe-low rpm highway hesitation/P0308 code
- Thread starter JayG
- Start date
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SunlitComet
OBS Jedi-Do Good
check the misfire history with the properly equipped scanner to be sure but it sounds like only the that cylinder is the issue. try a a efi solvent flush from local shop and have an injector balance test done. it may clear it up.
Thanks for the reply. I have already ran 2 cans of Techron injector cleaner thru it. Is that what you're referring to? Or something different? And I've never heard of an injector balance test. The injectors aren't accessible being under the upper plastic intake plenum. I'm aware of a cylinder balance test where you would short out the cylinder to diagnose if it were dead or not, but that's ignition related. Please explain further. And lastly, do you feel my fuel pressure is correct? According to many online forums it may be slightly low, but I'm not sure if being under by a few psi would be so critical, but some feel it could as these vehicles require 60 or better and I'm in the low to mid 50's psi when running. That's what kept me leaning toward either the spider or regulator possibly seeping. Thanks SunLitComet. Much appreciated. Really want to get to the bottom of this as it's frustrating the heck out of me.
SunlitComet
OBS Jedi-Do Good
no solvent flush is direct fed to injectors thru pressure test port with pump off and is much stronger. balance test is prime the pressure and watch pressure drop as a special test tool and adapter pulses the injector a given amount of time. you look for a pressure values based on diagnosis chart. i think your pressures are just fine right now.
Thanks SunlitComet for the advice. I have a shop in my area that I use when I'm in over my head or beyond my capabilities technically or tool-wise . I'm gonna give them a call tomorrow and mention my symptoms and the advice you gave and see if we can get this thing straightened out. I will report back. Thank you so much again !
96 Tahoe issues
Hi SunlitComet, I would've personally messaged but I haven't made enough posts yet to qualify. So I took your advice and brought my Tahoe to my local mechanic friend. We spent a couple hours going over it. He first hooked up his SnapOn scanner and although it was going between a P0300 code and a P0308 code when I had scanned it, it was coming up as a consistent P0300 code on his scanner. We kept it hooked up and drove it for a while at around the 2000 rpm level where the slight hesitation occurs and the scanner showed all 8 cylinders randomly misfiring consistently as the hesitation was occurring. The history showed the misfire on cylinder 8 around 3000 times and other random cylinders in the hundreds, thus probably throwing a P0308 code sometimes. We went back to the shop and tried power braking it in gear and it felt very sluggish. He decided to check the timing. I wasn't able to with my limited scanner. His scanner showed the timing was retarded at -14 degrees when held at 1000 or so RPMs. We advanced the timing to close to spec which I believe is 0 +/- 2 degrees. We were only able to get it to -2 degrees as the distributor wouldn't allow me to go counter-clockwise any further as the cap was hitting up against the upper plastic manifold plenum. This led my friend to think that back when I had the lower intake gaskets replaced that the mechanic may have installed the distributor off by one tooth, thus not allowing me to get any more advance. Regardless, we were at -2 degrees and took it for a ride with the scanner hooked up again. He cleared the code and no more misfires. We drove it a while and no more codes. Ironically the timing advance took care of the misfire. That's the good news. The bad news is I still have the symptom of the slight hesitation. We both now are pretty confident my original spider/regulator theory was incorrect, and that my symptom is not engine, ignition, fuel, or sensor related. By the way before seeing him I did a compression test too which showed all cylinders at or above 160 psi. So now we're thinking possibly torque converter trannny issue. Unless you can think of something else. I only have 70k on this truck which would really suck if it's trans related. I thought maybe even a brake hanging up might be a cause and I did pull the wheels today. My right rear has a slightly torn wheel cylinder boot but is barely leaving any residue. I would think something like that would be more constant than only on the highway at 2000 rpms. I will admit since we advanced the timing, correcting the misfire, the hesitation I feel now is definitely more subtle, but it is still there. Any thoughts or insight is greatly appreciated. Thank you again, JayG
Hi SunlitComet, I would've personally messaged but I haven't made enough posts yet to qualify. So I took your advice and brought my Tahoe to my local mechanic friend. We spent a couple hours going over it. He first hooked up his SnapOn scanner and although it was going between a P0300 code and a P0308 code when I had scanned it, it was coming up as a consistent P0300 code on his scanner. We kept it hooked up and drove it for a while at around the 2000 rpm level where the slight hesitation occurs and the scanner showed all 8 cylinders randomly misfiring consistently as the hesitation was occurring. The history showed the misfire on cylinder 8 around 3000 times and other random cylinders in the hundreds, thus probably throwing a P0308 code sometimes. We went back to the shop and tried power braking it in gear and it felt very sluggish. He decided to check the timing. I wasn't able to with my limited scanner. His scanner showed the timing was retarded at -14 degrees when held at 1000 or so RPMs. We advanced the timing to close to spec which I believe is 0 +/- 2 degrees. We were only able to get it to -2 degrees as the distributor wouldn't allow me to go counter-clockwise any further as the cap was hitting up against the upper plastic manifold plenum. This led my friend to think that back when I had the lower intake gaskets replaced that the mechanic may have installed the distributor off by one tooth, thus not allowing me to get any more advance. Regardless, we were at -2 degrees and took it for a ride with the scanner hooked up again. He cleared the code and no more misfires. We drove it a while and no more codes. Ironically the timing advance took care of the misfire. That's the good news. The bad news is I still have the symptom of the slight hesitation. We both now are pretty confident my original spider/regulator theory was incorrect, and that my symptom is not engine, ignition, fuel, or sensor related. By the way before seeing him I did a compression test too which showed all cylinders at or above 160 psi. So now we're thinking possibly torque converter trannny issue. Unless you can think of something else. I only have 70k on this truck which would really suck if it's trans related. I thought maybe even a brake hanging up might be a cause and I did pull the wheels today. My right rear has a slightly torn wheel cylinder boot but is barely leaving any residue. I would think something like that would be more constant than only on the highway at 2000 rpms. I will admit since we advanced the timing, correcting the misfire, the hesitation I feel now is definitely more subtle, but it is still there. Any thoughts or insight is greatly appreciated. Thank you again, JayG
SunlitComet
OBS Jedi-Do Good
you have no control over timing. only the pcm does. your soul task in it is to set the cam retard to zero as best as possible. do you fell you have achieved that?
When my mechanic thought the timing was off, he got on his computer to bring up the proper procedure for checking it with his scanner. He followed the instructions precisely and we did it together. Like I mentioned when we first checked it the cam retard was at -14. We were able to adjust it to -2. So yes, I do feel we achieved the proper setting if not very close to it. It obviously was an issue as it was creating a misfire and after resetting it corrected the misfire and no more codes. I'm still curious as to why my distributor suddenly was out of whack though. The hold down bolt wasn't loose. I've read where there are issues with the distributor reluctor (I think that's what it's called) on these vehicles. I wonder if that may be something to look into. But either way the misfire is cured. Just don't know where to go from here. We keep leaning toward trans torque converter. My mechanic actually called his friend at our local chevy dealership and mentioned our situation and the guy said if there aren't any more codes or misfire issues and there isn't any mechanical issues with the motor that the transmission valve body can be acting up causing the torque converter to shudder. He said that it's also a common issue with these trucks. I have to say I'm a GM guy for life, but I am really disheartened with the amount of issues I've had with this vehicle since I bought it. It's virtually in showroom condition with little miles and been pampered. But I've done intake gaskets, water pump, AC compressor, alternator, fuel pump. Not to mention the other standard tune up and maintenance. It's crazy. So what do you think my next move should be? Again thank you very much for your insight !!
SunlitComet
OBS Jedi-Do Good
no dizzy issues. give your plugs a good once over to see if one has gotten fouled up bad from the misfiring causing it to still fail under a high load. same with the wire and dizzy cap.
if you unplug your brake light switch and the problem is still there it is not a converter issue as you just disabled the lockup system. warning if you drive like this you also have no brake lights during your testing. might get a ticket.
btw what brand of ignition components you got.
if you unplug your brake light switch and the problem is still there it is not a converter issue as you just disabled the lockup system. warning if you drive like this you also have no brake lights during your testing. might get a ticket.
btw what brand of ignition components you got.
Cap, rotor, plugs, and wires are all AC Delco a couple months old as well as the fuel pump and sending unit a few months old. I will check the plugs again later this week, but last week or so I had them out to do the compression test and they all looked fine. And shortly after that when my mechanic had adjusted the cam retard up to spec, we drove it a while after we cleared the codes, and while watching the SnapOn scanner there were virtually no misfires occurring on all of the 8 cylinders while we were driving around. So I'm pretty sure the ignition components are good. The last time I had the truck out trying to rule out torque converter issue, I very lightly laid on the brake and I'm pretty sure the slight hesitation was still there. It was kinda hard to tell so maybe I'll try a short run with the brake switch disconnected to get a better judgement call on whether it's still there or not. Again I really appreciate you trying to help me figure this out. I will report back later this week with my findings.
