I replaced the front exhaust and lower intake manifolds this weekend, and now I have a major and very consistent miss under any load. It runs perfectly in park, but shakes and sounds like a helicopter once in gear, and at all speeds.

I've tried everything i can think of to solve this problem. Computer checks out ok in both the KOEO and KOER tests. fuel pressure is 50psi with key on and running, and stays steady while driving. I replaced all 6 spark plugs and wires, the cap and the rotor. I used a can of starter fluid and a can of carb cleaner trying to detect manifold leaks. I also checked the vacuum lines, and replaced a few that seemed worn.

A vacuum test shows good vacuum at idle but drops way down under a load. Someone at an auto parts store told me this was normal, but I'm no sure. If i do have a manifold leak, its seems only to leak under a load and that does not seem to make any sense.

The van ran very well before this botched repair, so i don't think my problem is compression or what have you.

I also swapped out the egr valve with a spare, no change. I disconnected the exhaust manifold at the pipe to see if the converter was plugged, no change. I pulled all the spark plugs after a few hundred miles like this, trying to find the missing cylinder(s), all looked clean and good.

Anyone ever had this problem? I'm considering a stick for the gas pedal and a very tall cliff.

 

Did you check the new exhaust manifold for blockage before you installed it? It's not unusual for one to arrive with something shoved down in it.

 

Your fuel pressure suggests a vacuum leak, should be 58psi.

How segnificant is the drop under load? If anything during acceleration vacuum should increase as velocity through the manafold increases not drop. Again, this suggests you've got a vacuum leak.

Since you replace the lower intake double check all vacuum lines and specificly test around all intake mating surfaces. The poor performance, lower fuel pressure and odd vacuum drop under load strongly suggesting that your sucking extra air in someplace downstream of the plenum chamber. I'm a little suprised that your not getting a too lean error from the computer.

 

It should be 55-60 with the key on and the engine off. A vacuum leak should cause the pressure to rise, not drop. 50psi is good when the engine is idling.

Ported vacuum increases with airflow, manifold vacuum drops under load. EFI engines don't use ported vacuum because it is most often used to control vacuum advance with a distributor. Under any appreciable load the manifold vacuum should drop to zero or very close to it indicating that the throttle body is giving the engine all the air it needs.

If your fuel pressure stays at 50psi while driving that is probably your problem. It should rise to 55-60 under load. The increase in fuel pressure is like the accelerator pump and powervavle on a carb all rolled into one. Without that it will run lean.

 

New manifolds? Or just new gaskets?

--check your sparkplug wires again.

--check the intake for cracks.