Question about 1998 GMC Yukon

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Engine misfires when traveling up long steep grades and hignway speeds. Problem happens when transmission downshifts to maintain speed.

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Could be a couple of things. Have you changed plugs, or plug wires. Suggest you do if engine has lot of miles. Could be a (the) ignition coil. Take your GMC to Auto Zone, they will hook it to their computer to diognose, for free. It will tell you exactly what the problem is

Posted on Jun 04, 2010

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Posted on Jan 02, 2017

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Car surging


Is the car equipped with the Automatic Transmission?
If so, read on.
Otherwise skip to the end and answer some questions.

POSSIBLE SCENARIO:
I have observed a condition where my car surges slightly when the torque-converter clutch (TCC) cycles between lock and unlock when driving on an uphill grade.
First some basics and history that will explain why the TCC is used.

Engine, Torque Converter, TCC, and Transmission relationship--
The TCC allows for a solid connection between the engine and transmission which allows the input to the transmission to rotate at the same speed as the engine.
Without a TCC, there is slippage between the engine and automatic transmission. The slippage is greatest at low engine RPM. That is what allows the engine to run with the automatic transmission in gear, like when you first shift into gear or stop at a stop sign. When the throttle pedal is depressed, the engine RPM begins to increase and the torque converter begins to slip less and less the more the engine RPM increases. The car moves. But even at cruising speeds the torque converter slips slightly. Engine RPM is greater than transmission input RPM, which is realized as slight decrease in fuel efficiency.
When acceleration is complete and a constant speed is being maintained, the engine power output is reduced to the point where the TCC can engage and eliminate any slippage between the engine and transmission. If the car has a tachometer the engagement of the TCC can be verified when a slight reduction in engine RPM observed without a corresponding change in vehicle speed.
One method used to test the operation of the TCC is as follows:
Find a flat section of road where it is safe to perform the test.
Reach a steady speed and keep the gas pedal depressed with one foot. While observing the tachometer (or listening for an increase in engine RPM), with the other foot depress the brake pedal enough to activate the break light switch but not enough to engage the brakes. When the brake light switch activates, the TCC receives a signal to disengage. With the gas pedal being held steady, release the brake pedal and the engine RPM should decrease when the TCC engages.
Old cars with Automatic Transmissions did not use a TCC. I believe the TCC was put in use in an attempt to increase fuel economy.

MY EXPERIENCE WITH SIMILAR SYMPTOMS
The condition that causes that issue on my car is this:
- A slight uphill grade increases the load on the engine.
The car tends to gradually slow and it is necessary to depress the gas pedal to maintain speed.
- Depressing the throttle pedal (manually, or automatically with cruise control engaged) signals the torque converter clutch to unlock when the load increases slightly. (A more drastic load increase would signal the Transmission to downshift to a lower gear.) The corresponding increase in engine RPM and output is enough to compensate for the reduction in speed. When the vehicle speed, engine RPM, and throttle position stabilize to the point that the TCC will engage and the engine RPM will reduce in correspondence with TCC engagement. Now, if the road conditions have not changed, power output is not enough to maintain vehicle speed. With the increased load caused by full engagement between engine and transmission, and the cycle (surging) repeats itself until the road conditions change.

Does that help?
If not:

QUESTIONS
Please define the symptoms.
What are the road conditions when the surge occurs? (A slight uphill grade?)
What is the frequency of the surge?
Does the engine power output have a noticeable surge?
Is there a speed change related to the surge?
Does the tachometer move up and down with little or no change in vehicle speed?
Are all instrument indication in the normal range?
What else has changed?

Good luck!

May 24, 2014 | Subaru Impreza WRX STi Cars & Trucks

1 Answer

I have a 2001 Silverado HD with the 6.0 gas engine, 245,000 miles on it. At highway speeds, the check engine light blinks. Not constantly, but off and on throughout the trip. It never does it below 60 mph,...


engine lights report on the engine AND the transmission faults
could be a problem wiring harness connection to the transmission as well as the engine
from your statement, it may be having a low fuel pressure problem at speed when the requirement for full fuel flow is required so check the fuel pressure ( 45-65 psi)
take it to an accredited transmission professional service center and have the transmission check ed out as well
it may cost for the diagnostic tests but with quotes for repairs at least you will have knowledge before hand

May 10, 2017 | Chevrolet Cars & Trucks

1 Answer

1999 silverado transmission


Since the transmission shifting and torque converter lock (cruise speeds) up are computer controlled you need to have a scanner hooked up that can read Transmission control module (TCM) fault codes. Also check for engine codes as this could be a misfire, but in that case the check engine light should be flashing while the engine is misfiring and set a fault code. If no codes of any kind are present in memory when the testing is done you almost certainly have a mechanical fault in the transmission like a malfunctioning torque converter clutch, but even that should set a fault code, so have the testing done.

Mar 23, 2017 | Chevrolet Silverado Cars & Trucks

1 Answer

2004 Hyundai Sonata automatic transmission downshifting while at a maintained speed.


I have one that does exactly this. I have confirmed no codes in trans or engine; I have watched the data on the scan tool and it appears that the PCM is choosing to shift without a change in throttle or speed to cause it. I am looking into a new PCM - ouch!

Dec 19, 2012 | Hyundai Motor 2004 Sonata

1 Answer

I have a 1997 lumina with 3.1L. the problem started with loss of power under load and p0300 code. check coils and vacuum leaks both OK.change spark plugs and wires.[same problem] check compression OK.


Since you changed the sensor we will assume it is working. When you disconnect it you put the computer in limp mode which stops it from using sensor values. If code 300 is the only code we will assume the coolant temp sensor and others are ok.
I would check fuel pressure with a gauge next.

Aug 21, 2012 | 1997 Chevrolet Lumina

1 Answer

I have a 2005 Dodge Ram 1500 Quad Cab 2wd w/4.7L Engine. My transmission experiences two problems. Problem 1 is that sometimes (not always) when letting off the gas at a slow cruising speed the...


Try some tranX. My transmission would drop out from 2nd and 3rd , I put this in and with in 3 days it quit slipping. and shifted normally. It's been over 2 years now without a problem

Nov 27, 2010 | 2005 Dodge Ram 1500

1 Answer

2000 Ford Explorer A/T downshifts unexectedly


The vacuum modulator on the side of the transmission has a leak (rubber diaphragm inside has ruputred. ($15 AutoZone).
Or other major vacuum leak (hose to the modulator may had come off or brittle, or clogged, or broken.

The downshifting is now being done by the "mechanical backup" to the vacuum system

Hope this helps!

Sep 13, 2009 | 2000 Ford Explorer

1 Answer

1994 BMW 540i does quick, short, unexpected downshifts


Dear sea_lion42, Friday 2-20-2009 11:22pm EST (USA)

Check your transmission fluid which is usually done with the engine warm and running!
If the tran fluid is low add some Lucas Trans Fluid from your local auto parts store!
The knock makes me believe your low on trans fluid!

jack
3437e84.jpg

Jan 23, 2009 | 1994 BMW 5 Series

1 Answer

Car loses power


When the check engine light is flashing that indicates an engine misfire, which is catalyst damaging. Because you said that it generally only happens up a long hill or at higher speed, and it doesnt happen all of the time, most likely you are in need of a full tune up.

Dec 11, 2008 | 2003 Hummer H2

2 Answers

Misfire & power loss


maybe its time to replace your fuel injectors or fuel filter ?

Aug 05, 2008 | 1999 Chevrolet C1500

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