So here's the deal. I have a 96 4.3 with 108,500 miles and a lot of problems. Recently, I replaced the thermostat and now it seems like it takes forever for the car to warm up and get out of open loop. When I first start it and let it warm up for a bit, I'll go a few miles and when I get to 4th gear/elec. OD, it starts to buck and stumble. If it happens bad enough, the emergency brake light and the ABS light flash on for about 3 or 4 seconds like it's going to stall. I do notice that after the truck has warmed up, this problem occurs less frequently. Occasionally, it will totally stall out after it bucks. It also will go in and out of the electronic overdrive while cruising at around 50mph. I've also had problems with the EGR valve(P1406) (replaced it) and the TPS(P0122) (replaced it) but the codes keep coming back after I clear them with my scanner and take off the neg. battery terminal. Does anyone have any idea of what I can do to start to figure out a problem? It will have these symptoms for a while then it will go away for a while. The other weird thing is that I used to clean the Idle Air Control Valve and the bucking would go away for a while. I have since replaced the valve and maybe it was just pure chance. who knows. This has been bothering me for some time now! please help! Thanks!

 

bump Please Help

 

Sorry, must have missed this post with the errors the forum has been having lately.

These two problems are most likely related. And they could possibly be caused by a faulty PCM.

P0122 Explained
P1404 Explained - now I know that yours is P1406 which deals more with the overall circuit performance, but P1406 is not listed in my diagnostic trouble chart listings. P1404 pertains to the closed performance, but goes through almost all of the relevant checks that can be performed on the EGR system for problem verification.

I'm betting that the problem is a loss of the 5V reference voltage from the PCM that gets passed to these two components for monitoring of critical engine functions. It could also be the grounds for the PCM are corroded... You should check both the 5V reference at the TPS and EGR valve as well as the continuity to ground at the connectors of each. If you get anything but a high continuity to ground, I would suspect poor PCM grounds.

The TPS isn't that expensive, but the EGR is and I sure wish you had asked before you replaced it/them. Could have saved you some money!

Run through the checks that you can do (even if that means tapping off the 5V reference with a volt meter and driving down the road!) and let us know!

 

It's ok, I appreciate the quick response. So I'll check the grounds tomorrow hopefully, and see where that gets me. In regard to the bucking and the ABS light, do you think that it would be a sensor causing the abs to come on and cause the engine to buck like the brakes where being applied for a split second? Can I have the ABS codes read even if the light isnt on? Thankss

 

The ABS and brake light coming on could be further evidence of either a ground problem or a PCM problem. These lights are controlled by the PCM so it could be a hard fault somewhere in the PCM that is causing this.