Part throttle buck on 99 4.0???

[Heinriech]

A co-worker has a 99 4.0 Mazda B4400. In the past year or so he has replaced:

Plugs
Wires
Air filter
1 of the 2 upstream Oxygen sensors
Cleaned the MAF 2 weeks ago

He recently had the truck in for service because of a check engine light and they replaced the intake manifold gasket.

Symptoms are only at cold startup and when cruising and steady on the gas.

Any ideas?

Thanks

 

[jasonl]

What was the check engine light code? How bad is it "bucking"? A bad egr sensor (dpfe) can cause surging at low rpm cruising .

 

[Heinriech]

P0401 (EGR malfunction)

Ok so do a vacuum check on the EGR valve and possibly clean it? Or just replace it? He says the sensor looks new with new soft lines running away from it.

Edit: I don't know how bad the buck is but he told me this morning that its getting worse.

Thanks!