SES Lamp - Multiple Engine Misfire!

[usscvr]

OK - I have a yet undiagnosed problem here....

I have a 99 Yuk 4WD w/ 110K miles. At around 80K miles, I noticed that the SES lamp came on at Hwy speed while under a load (towing, going up large hills at 70-75+ mph) when the engine had to work harder to maintain speed. There was nothing noticeable at first except the SES lamp. I checked the codes and a "Multiple Engine Misfire" code came up. I changed the Cap, Rotor, Plugs and Wires, thinking that was the problem. Didn't work. It kept doing it.

Later, when the SES lamp would come on at Hwy speed, still under a load, it was obvious that the ECU would change the ignition timing b/c the engine would have a SEVERE vibration, massive loss of power, etc. If I backed off the throttle a bit, it would "re-set" itself somewhat and the vehicle would stabilize. I checked the codes and the same "Multiple Engine Misfire" code came up, as well as "Misfire #4" from time to time. Frustrated, I took it to the dealership. They performed the same Cap, Rotor, Plugs fix. Didn't work. The second time to the dealership they suggested an Upper End Cleaning. The lamp came back on the way home that very day!

Now, when it happens, still on the Hwy at speed, the SES Lamp flashes (b/c it’s on all the time now) and the engine vibrates, and the tranny hard-shifts after coming to a stop and starting again. If I turn the vehicle off, wait a few seconds and re-start it, the tranny and the engine appear to “re-set” and operate normally again. I am really frustrated with this.

Of course I have asked around, and I’ve had a couple of people suggest that it might be a Fuel Injector problem, but I have a throttle body FI system, not individual injectors. I think it’s a sensor or something, or a couple of sensors, but the small shops and the Dealerships cannot seem to find out the cause. Can anyone help? Please post a reply, or email me directly at [email protected]. I need all the info I can get!

[carsonsig]

I dropped you an email!

[whoppo]

My 99 Escalade started doing the very same thing at around 80K miles..
it got to the point where the SES light was always on and the symptoms got worse. I finally managed to get a day off to bring it to the local Caddy dealership for a way-overpriced diagnostic. The computer codes showed a fuel flow/mixture problem and a pant-load of misfires on one cylinder. The tech knew right away that I needed an new EGR valve.. he tested the valve and sure enough, it would only open 13% of the way. In went the new EGR valve, out went the SES light and the truck has been running perfectly since then.

Hope this is of help.

Cheers!

[usscvr]

OK guys - UPDATE. Now my codes show "Catalyst Efficiency Below Threshold" in addition to the previous codes of "Multiple Engine Misfire" and "Misfire #4." Does this sound like an emission problem? EGR valve or O2 perhaps?

Thanks to all of you that have emailed me or posted a reply, from Spark Plug gaps to Fuel Pump problems. I'm still at a loss here.... can anyone help?

[hipp1981]

what was the problem