Engine surge on light throttle

70GTORAG · #1 03-25-2005, 11:18 PM

I am wondering if this is caused by a bad throttle position sensor. When there is light acceleration(throttle) only, the engine surges back and forth.
With vehicle in gear, brakes applied, and just a light touch on the gas pedal(take the engine to about 600 rpm on the tach), the engine will cycle back and forth from about 600 rpm to 900 rpm. If I apply more or less gas pedal the problem goes away. Does anybody agree with the throttle position sensor diagnosis, or is this related to the transmission?

Eggert · #2 03-26-2005, 08:27 AM

Hi,
my vote is for an engine management function being not sure on what to do. Almost like the boost you get when the A/C comes on. I would view the tranny as being innocent.
A diagnosis with the Tech II may be in order as there are so many sensor and computer interactions.
It usually goes: Send this command, check that voltage. Not much DIY any more.
Take care, Eggert

70GTORAG · #3 03-26-2005, 09:57 AM

Diagnosis with Tech II? - are you referring to the dealership?
Another question I have is will the ECM store trouble information even if the check engine light does not come on? This problem has not thrown the check engine light.

Eggert · #4 03-26-2005, 11:25 AM

Hi,
yes and may be.
Here is an ebay link for a tech II for $2,800...
http://cgi.ebay.com/ebaymotors/ws/eB...spagename=WDVW
Do you have an ODBC II DTC reader?
Take care, Eggert

70GTORAG · #5 03-26-2005, 11:41 AM

No I dont. Mine is ODBC I and I can ground the diagnostic terminal on the ALDL to retrieve codes but there is nothing stored. I assume that I will need to take it in for analysis and that the Tech II will provide voltage readings on the various sensors to see if they are operating within range?, yet not out far enough to set any trouble codes?
Are there any test I can perform at home before I take it in, such as the MAP sensor, TPS or EGR control solenoid. Thanks.

Eggert · #6 03-26-2005, 11:47 AM

Hi,
can you please post what year/model you have?

70GTORAG · #7 03-26-2005, 11:50 AM

Sorry, I should have posted that earlier. It is a 1994 Yukon 5.7 L.

Eggert · #8 03-26-2005, 12:25 PM

Hi,
I have no service info on that year.
I am sorry to say that Helm does not show a service manual for this year
http://www.helminc.com/helm/Result.a...5UJK866R349SP0
Take care, Eggert

70GTORAG · #9 03-27-2005, 04:15 PM

New information:
Borrowed the following sensors from identical vehicle to test in my 94 Yukon. TPS, MAP, EGR control solenoid. The problem was still there. I replcaed the Oxygen sensor last year. Did not check the engine coolant temperature sensor, but do not think this could be the problem. I am leaning towards the transmission now.
How is the transmission controlled, is there a sensor or does the ECM control the torque converter clutch?

2000CAYukon · #10 03-27-2005, 11:33 PM

Has EGR ever been replaced. A bad one will cause part throttle surging.

//2000CAYukon

70GTORAG · #11 03-28-2005, 05:25 PM

No. Truck is at 115,000 miles. Probably should have been done some time ago.
I had just assumed that a bad EGR typically causes poor idle conditions. However upon doing more research on the EGR system, it's function and effect are much more complex than I thought. I will test the EGR valve tonight and post results. This link provides excellent insight to the EGR system: http://www.motorage.com/motorage/art....jsp?id=101041

70GTORAG · #12 03-28-2005, 09:31 PM

Just finished replacing my pooched EGR valve, thanks for pointing me in that direction 2000CAYUKON.
All I can figure is that the return spring was weakened where it was allowing the diaphragm to move too much, therefore dumping too much exhaust gas into the intake, causing the drop in the rpm's to a point where the ECM signalled the EGR solenoid to cut off vacuum, returning the rpm back to normal. Then the cycle begins again.
Or, as simply said before, the EGR valve was pooched.

2000CAYukon · #13 03-28-2005, 09:38 PM

Quote:

Originally Posted by 70GTORAG

Just finished replacing my pooched EGR valve, thanks for pointing me in that direction 2000CAYUKON.
All I can figure is that the return spring was weakened where it was allowing the diaphragm to move too much, therefore dumping too much exhaust gas into the intake, causing the drop in the rpm's to a point where the ECM signalled the EGR solenoid to cut off vacuum, returning the rpm back to normal. Then the cycle begins again.
Or, as simply said before, the EGR valve was pooched.

Glad that was it. The EGR is subject to a lot of heat and they do wear out. My 96 Impala SS only had 45K on it when I bought it and it was the first part I replaced due to the part throttle surge.

//2000CAYukon

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