2004 Astro AWD - 120,000 miles
about three weeks ago I lost about 3/4 of the HP. At anything more than 1/4 throttle the engine falls flat on it's face and starts to spark knock
It acted like the exhaust plugged up. I tested for plugged catalitic converters - less than 1 PSI at idle and never saw more than 2 PSI when reved up. Changed the cap and rotor. changed the inline fuel filter ( it was plugged compaired to the new one) Just checked the fuel pressure at the port on the back of the motor. It was 53 at idle and went up to 60 at higher RPM's. I plan to change the plugs and wires after the engine cools off. Air filter is clean. I did have it scaned and the only code was for MAF sensor. I have one off of a good running 2000 ASTRO. This was done before changing the fuel filter. ( couldn't get the old fuel filter out untill two days ago with WD 40) The good used MAF did nothing.
This is a work van and I can't do my job without it. If I take it to a dealership they will just change parts untill they find out what is wrong. The new parts that get installed won't get removed even if they don't help.
HELP

Steve

Check the egr valve make sure it's not stuck open common problem on those. It will also cause an incorrect maf reading

I had that happen on the 2000 - it caused a very rough idle. Don't think that is the problem
It runs perfect and has good HP up to 1/4 throttle. Could it be the TPS what readings should I look for - It acts like it just isn't getting enough gas. If you feel the throttle it can be driven - just don't exceed the point where it fall flat. I can cruse 70 but looses speed at ANY hill.

make sure theirs no leaks in the air tube going from the maf sensor to the throttle body including the seal around the throttle body to the 90* air tube..
check tail pipe and see if its black with lots of carbon..if it is then your fuel pressure regulator ruptured and is dumping fuel into the intake ports under the upper intake.you will smell allot of gas...if none of this works or has any symptoms change the sparkplugs and wires..if yoyu havew a big plug gap the compression will blow the spark out....those vans are notorious for ingition coils also..i replace mine every 2 yrs just for a piece of mind..had my truck since 1996 and its onlt been on a back of a tow truck one time..

if none of this works also make sure the vacume line is still stsched to the front of the throttle body,,its rubber that goes to a plastic line that goes across the front of the engine towards the blower motor,those usually brake and will throw off the maf sensor..tps should read weith a meter unter 1v at idle and when you open the throttle body it should raise to the max of 5v at wot..

Thanks

04 rastro. vortec v6 with the spider-type fuel distributor inside intake? if it has that set-up, it is most likely problem.

What he said !

doubt it,,,the poppets either drop a cylinder or two or blow the ecm fuse from a dead short...

The engine starts perfect and idles the same. It runs great up to about 1/4 throttle and then falls flat.
It is not running rich. I believe it is running lean at higher RPM's.

Tps should be .48 at idle and 4.6-4.9 wot, check fuel pressure running down the road, I've seen some gm pumps test fine in the shop but can't supply the volume under load. And check everything fix all posted he is right on with all of that