This problem’s too modern for me

Question:

A very good friend of mine has a Mopar with a mid-range bog and we’re really taking a beating trying to track down the source of the problem.

He says it runs fine at idle but can’t get going from a stop. I asked him if the exhaust was plugged and he replied it was not hooked up from the Y-pipe back so it’s open.

I forgot the year of the vehicle, but know it is a throttle body and it’s the first year of that on the 318 and it is a pickup. He said he’d had the fuel pump in a bucket and it really pushed the petrol so I didn’t go there. I asked about whether the engine has spark and he said it starts and runs well.

I’m at a loss because I didn’t follow the throttle body fuel injection movement. I still like the old carburetors. If it was a 1946 Chevy I could tell him some stuff. Could it be something as small as a bad ground? Mopars were great for that in the later 1960s to the ’70s.

Answer:

When diagnosing problems with computer-controlled engines, the logical starting point is determining whether or not the computer has recorded any trouble codes. A trouble code usually will not pinpoint the exact cause of the problem but it will get you pointed in the correct direction.

Speaking in general terms, some of the things that can cause a mid-range bog with your friend’s engine include mismatched parts (for example, are the throttle body and intake manifold an OEM combination?), a bad engine control computer, a vacuum leak (look extra hard at the line that connects between the throttle body vacuum port and fuel pressure regulator), a bad throttle position sensor, a bad MAP sensor, or a bad fuel pressure regulator.

Irregular fuel pressure also can be a problem and watching what comes out of the pump when it’s laying in a bucket of fuel is not a valid way to test the function of the pump.