What might cause engine shudder at initial throtle opening? on 1999 Ford E-250

No fault codes set. Intermittantly runs rough on initial throttle opening only. I replaced the coil pack, all the vacuum hoses, ignition wire set, spark plugs, cleaned throttle body. Condition persists. I am thinking maybe a faulty ER valve or TPS, but they're not easy to get to so I'm holding off until I can get other opinions.

by in Summerville, SC on May 17, 2012
1 answer 1 comment
ANSWER by on May 17, 2012
Any CEL on? That is one of the first things you should tell us. May have a motor mount problem. Was all these parts replaced because you were trying to fix the problem you have stated?
COMMENT by on May 17, 2012
I replaced the parts because soon after I bought it late last year, I noticed this condition and performed catch up maintenance. I replaced all the ignition parts becasue the installed parts, with the exception of the spark plugs, were all original. I scanned it to be sure, but it is not setting any input/output codes. No, it's not motor mounts, it is throttle related. I operate a one unit on call service business, so my truck is full of noisy tools. When the shudder occurs, the entire truck vibrates until throttle opening is increased. There is another poster here with the same problem. It only does it on initial throttle opening during acceleration enrichment. Beyond that throttle position, it runs perfectly.