RPM Surge 40-57 mph
#1
11-09-2018, 02:30 PM
RPM Surge 40-57 mph
Long time lurker, first time poster. This place has been the authority for all of my maintenance and troubleshooting ever since I got my vehicle. I’ve seen posts about this before, and replaced a slew of different things mentioned on the forum, but to no success. I decided to register and make a post in case anyone else has experienced and solved this issue.
1998 GMC Jimmy SLE 2WD. 230,000 miles
Starts, idles, and runs great, no service engine lights, and I keep up pretty well with regular maintenance and fix other issues as they come to me.
Drives good, shifts good, but when I hit about 40mph and it goes into OD, the RPMs fluctuate up, then back down. Up then back down again and again. The jump in RPM is more significant as my speed increases, and then totally stops and acts normal when I reach 58 mph. I do a lot of driving for work, so it happens everyday, and I would like to solve this problem so I don’t hurt the motor any more than I may already have. I’ve been experiencing this for about 2-3 years, and cannot seem to pinpoint where it’s coming from or what to do.
I’ve replaced the throttle position sensor and fuel filter, checked for blown fuses and vacuum leaks. New AC Delco cap and rotor, new plugs and wires, and EGR valve.
This all began after I was involved in a front end collision and had to replace the fenders, hood, grill, bumper, radiator, and fan. This could be an important detail but I’ll let you decide. Please help me out and thank you all in advance.
1998 GMC Jimmy SLE 2WD. 230,000 miles
Starts, idles, and runs great, no service engine lights, and I keep up pretty well with regular maintenance and fix other issues as they come to me.
Drives good, shifts good, but when I hit about 40mph and it goes into OD, the RPMs fluctuate up, then back down. Up then back down again and again. The jump in RPM is more significant as my speed increases, and then totally stops and acts normal when I reach 58 mph. I do a lot of driving for work, so it happens everyday, and I would like to solve this problem so I don’t hurt the motor any more than I may already have. I’ve been experiencing this for about 2-3 years, and cannot seem to pinpoint where it’s coming from or what to do.
I’ve replaced the throttle position sensor and fuel filter, checked for blown fuses and vacuum leaks. New AC Delco cap and rotor, new plugs and wires, and EGR valve.
This all began after I was involved in a front end collision and had to replace the fenders, hood, grill, bumper, radiator, and fan. This could be an important detail but I’ll let you decide. Please help me out and thank you all in advance.
#2
11-09-2018, 02:55 PM
Shot in the dark, but you did not mention transmission maintenance. Could something be slipping in there? Could some speed sensor not be working correctly?
#4
11-09-2018, 04:38 PM
Torque converter and transmission sensors were some of my next thoughts but I don’t really know what to change out. I also considered crankshaft or camshaft sensors but I wanted to ask around and see if I could rule anything out before I start replacing parts that won’t fix the issue.
#6
11-09-2018, 07:12 PM
Upon further research, I don’t think my truck actually has a TV cable to adjust, I believe the transmission is electronically controlled. But with slack in my throttle cable, maybe it’s giving the TPS (throtttle positioning sensor) an inaccurate reading and maybe that has something to do with it. Could the TPS have anything to do with telling the transmission when to shift?
Last edited by Bullfrog; 11-10-2018 at 07:37 AM.
#7
11-09-2018, 08:45 PM
Know its different, but I had a Ford that did the same thing. Trans was good, but it was the Coil. On it with Coil over plugs. It had a coil for each plug. One coil was weak, when the EGR was putting gases back into the engine. That coil was called upon to fire hotter to ex-nite the low Oxygen fuel mixture.
Thus it would make you feel the Trans was going out. My last Vortex was in a 99 S10 pickup.
So lock the Blazer out of OD. And if it does not do what your feeling. It has to be tied into the EGR gases returning into the engine. The ECM calls for a hotter spark. When the EGR is putting the gases back into the system. So you have one coil, that fires fine. When it has plenty of Oxygen and gas. But when the EGR puts low Oxygen gas's back into the system. It does not fire hot enough to burn the gas right.
I had to swap one new coil. In place of each one, until I found the bad coil. It would not set a code telling me what bank or coil it was. Just one had gotten weak.
Hope this helps you out. Most may not know a weak coil will case this issue
Thus it would make you feel the Trans was going out. My last Vortex was in a 99 S10 pickup.
So lock the Blazer out of OD. And if it does not do what your feeling. It has to be tied into the EGR gases returning into the engine. The ECM calls for a hotter spark. When the EGR is putting the gases back into the system. So you have one coil, that fires fine. When it has plenty of Oxygen and gas. But when the EGR puts low Oxygen gas's back into the system. It does not fire hot enough to burn the gas right.
I had to swap one new coil. In place of each one, until I found the bad coil. It would not set a code telling me what bank or coil it was. Just one had gotten weak.
Hope this helps you out. Most may not know a weak coil will case this issue
Last edited by tnpete; 11-09-2018 at 08:53 PM.
#8
11-10-2018, 03:53 AM
If you have ODBII live data capability you can ride with someone to monitor the live data for sensors, ignition, fuel trims, etc to see what’s starts drifting. You can also tape a fuel gauge to the windshield to make sure fuel pressure does not get wonky.
George
George
#9
11-10-2018, 07:31 AM
Any thoughts on what sensor/solenoid it could be? I’ve done a little research to get an idea of which could be causing this, but I don’t know where to begin. The VSS (variable speed sensor), the TCC (torque converter clutch) solenoid, are both places I thought to look, but I figured if those were bad, I would get a code reading. I’ve neglected to change the tranny fluid and filter for awhile now because I wanted to take care of anything else in there that might be causing problems while I already have the pan dropped.
#10
11-10-2018, 12:08 PM
Ok well I may have done something different today to help us narrow this down. I had read here on the forum somewhere that if the RPMs are surging, then try lightly stepping on the brake. I did that today and while I was in the RPM surge range, I stepped on the brake and my RPMs stabilized, while coasting and while under acceleration.
My memory from the post is foggy but I think I read that activating the brake like that, tells the torque converter or transmission to do something different (can’t remember exactly what). I also think I had read that someone figured out how the rewire or bypass this to make it think the brake is always activated so it will stop doing this (again I can’t recall exactly).
Any help or suggestions would be great, as this has been boggling my mind for a few years now. Thanks.
My memory from the post is foggy but I think I read that activating the brake like that, tells the torque converter or transmission to do something different (can’t remember exactly what). I also think I had read that someone figured out how the rewire or bypass this to make it think the brake is always activated so it will stop doing this (again I can’t recall exactly).
Any help or suggestions would be great, as this has been boggling my mind for a few years now. Thanks.
Last edited by Bullfrog; 11-10-2018 at 12:10 PM. Reason: Forgotten detail