MAP Sensor Question

I just built a new 350 for my neighbor’s 1994 Chevrolet Z71 1500. We put it in, got it timed, got the valves adjusted, and cranked it up. It runs great IF the MAP sensor isn’t plugged in. It won’t idle if it is. The throttle body is providing the vacuum it needs. So I figured that the old MAP sensor was toast and changed it out, but the problem isn’t going away.

The problem is, when the MAP sensor is unplugged it trips off the engine light and makes the truck run rich, so we can’t pass emissions. The light goes off when the MAP sensor is plugged in and the truck won’t idle, but when its unplugged the light comes back on but it runs better instantly, so I can’t test the OBDI system for any other trouble codes other than “MAP Sensor” when the MAP sensor is unplugged. We also changed out the Idle Air Control and the EGR vacuum solenoid, and the EGR valve thats on it is only about a year old, so it should be good still.

Any ideas? I have never had a problem with a 350 like this before.

Sorry to reply with a string of questions here, but:

Is this a TBI engine or a MPFI? Is it a rebuild of the old engine or an identical engine or is there some interchange going on? If so, is the computer from this truck or the donor vehicle? Are all the other sensors new, from this truck or from the donor vehicle? When you determined that there was vacuum, did you actually put a vacuum gauge on it, and if so how did it behave?

I assume you would have checked this, but have you checked the line that runs to the MAP sensor? I’ve had that get clogged on one of my rigs a few times.

Thanks for the reply, I super appreciate it. I have sunk almost two weeks of my free time to work on other things into this badboy. Its an identical motor that replaces the original. The original block had 423,000 miles on it, so it was too worn to mess with, but the numbers from the block and everything matched.

Its the original ECU, it tested fine.

All of the sensors are new (now) except for the MAP sensor itself. My neighbor is really having a tough time economically so I have been buying these parts for him to help him get back on his feet. I did buy the new MAP sensor, but since the old one actually tested fine and it acted the same way as the older one, I took it back. But the other sensors are new and test properly.

I did the vacuum test using the finger method. I couldn’t find numbers on how hard it should be sucking at base idle, but its sucking pretty hard. I can blow through the line with no problem, and I did the “spray carb cleaner around to see if you can find the vacuum leak” test and didn’t find any. I am going to hook it up to a vacuum pump tester when I get home tonight, does anyone know what the vacuum should be at idle for a 350?

The thing has a TBI (throttle body injection) system. The TBI is from the original engine, but it tested well and doesn’t leak. I cleaned it and rebuilt the gaskets in it, and it does well when the MAP sensor is unhooked, besides running a bit rich as you would expect.

I am kind of leaning toward the EGR valve being stuck open, but its new. I have bought new ones before that have been bad, so I guess its possible. I am going to check when I get home to see if its stuck open and can be cleaned or fiddled with to get it to operate properly if its being funky. I don’t want to buy another one of those because I can’t find the receipt on the one I have and they are almost seventy dollars.

I can post a video of what its doing if that will help as well. Plus, you can see how blue and pretty it is. But really, if that will help I can get that put up on YouTube or something. I am really trying to help this guy out.

Almost any car will read 15-20" of vacuum at idle (maybe a little higher at fast idle), 17-18 at curb idle would be textbook ideal.

Disconnect the vacuum hose to the EGR valve. I’m pretty sure it should have no vacuum at idle. If it does, and disconnecting it solves the problem, then it’s a matter of troubleshooting the EGR controls (the solenoid would be the prime suspect).

Next, remove the EGR valve and find some way to plug its passage to the intake. If that solves the problem, almost certainly it’s a leaking EGR valve. If it still won’t idle, the problem lies somewhere besides the EGR system.

My truck has had the same problem when I start it up it runs rough for 10 seconds then clears our an check engine light comes on but runs normal just really rich. changed aic sensor, tps , checked egr checked for vacuum leaks did notice when I sprayed carb cleaner around egr the engine acted like a vacume leak then leveled out. But we also changed out tcs by the thermostats housing. Nothing changed until I unplugged the map sensor the truck run better instantly an now will start normal but still a little rich an check engine light is on. Pls help so aggrivated