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94 Chevy 350 tbi shudders in gear

I bought a 94 chevy 350 tbi and finnally got it to idle right. i rebuilt the top end and have replaced my entire ignition system except the ignition coil. it idles great but if i put it in any gear except neutral and park it will start shuddering badly. i have no idea what to do except map sensor. this is my first car and first big project

If this is your first car, you’ll need to know how to pull the trouble codes from your OBDI GM vehicle.


Check for a misfire by shorting out the plugs and use a vacuum gauge to check for a vacuum leak. If you isolate a miss and the vacuum indicates a bad valve you likely adjusted a valve too tight.

The rough idle might be caused by a dirty/faulty Idle Air Control valve.


I borrowed a code reader
from a friend but haven’t gotten around to plugging it up yet

If it was a misfire wouldnt it also do the zame at idle and when i rev it up?

Not necessarily, it might fire under low load, but adding any load makes it harder to fire.

I have a Corolla of similar OBD I vintage. If I had that problem I’d start by making sure the idle rpm, idle ignition timing, and idle fuel pressure are correct. Are you certain the spark plug wires from the distributor are connected to the correct cylinders?

I can definitely double check tomorrow

How do you go about testing fuel pressure on the tbi?

i bought a new one and cleaned it all with tb cleaner. anything else i might could try

Don’t know, never done it on that engine. The service data for the suburban 5.7L says to install (presume it means “t-in” ) a fuel pressure gauge at the outlet of the fuel filter. Starting the engine should show a pressure of 9-13 psi. That’s a pretty low pressure, but apparently that’s what it is supposed to be. That low of designed-in fuel pressure is gonna pose you problems indefinitely I expect. Fuel injection systems tend to clog unless the fuel pressure is quite high, 35-60. I owned a mechanical injection VW Rabbit, so low fuel pressure injection systems is something I have had some unpleasant experience with. If you want to do a project some day, see if it is possible to upgrade to a port injection system with higher fuel pressures.

I wasnt able to use code reader as it was the wrong one. I managed to get codes by jumping the two right top ports where you would plug up code reader. I got one for the fuel pump, map sensor and shift solenoid. im guessing map sensor is causing the power lost and would the shift solenoid to shudder when put in gear?
Im also going to try to get what i need to test fuel pressure

Any other suggestions. Thanks everyone

The map sensor has to be working for the computer to figure out the correct air/fuel mixture for the driving situation. If the map sensor isn’t working the computer probably makes an educated guess for the mixture, which would allow you to still drive the car but expect some drivability symptoms. map sensor are quite expensive & failures are very uncommon as reported here, so double check the electrical connection to the map sensor first, then use a hand-held vacuum pump to see if it holds vacuum or not. It should. The shift solenoid has something to do w/the automatic transmission and could cause symptoms such as this, esp if it isn’t dropping to a low enough gear for the situation. The torque converter lock-up solenoid failing could cause that too. I’d guess the fuel pump is the prime suspect at this point. If the fuel pressure is incorrect there’s no easy way for the computer to compensate.

What were the actual numeric codes? I don’t think there’s a “fuel pump” code, per se. Those trucks have a fuel filter outside the tank. It’s located in line with the fuel line on the frame rail. They’re easy to change, and if you do have low fuel pressure, I would try that first.

I got 33 forthe map sensor (however i understand a lot of things can cause that code) 82 for the shift solenoid and 54 for the “fuel pump low voltage” or from what i just read could also be an egr problem. Plan to clean and check egr tomorrow and also check map sensor and possibly eliminate one of those

Hmmm…I’m not sure what would cause low voltage to the fuel pump other than the fuel pump relay or the wiring itself. Does the truck run well other than the poor idle in gear?

You said you rebuilt the top end. What does that mean exactly? Did you replace the intake manifold gaskets and have the heads reworked? I’m wondering if that code 33 might be caused by a vacuum leak.

Codes aside, it might be a good idea to clean the throttle body a bit - mainly clean the idle air control sensor pintle and it’s bore in the throttle body. Also clean the port on the throttle body where the map sensor attaches and the rubber elbow between map sensor and port. Make sure the elbow isn’t cracked or blocked. Check the vacuum hoses for leaks. Most of that is general tune up stuff that you may have already done, though.

A couple of shots in the dark you might try.

-You can swap a known good relay of the same kind for the fuel pump relay. Might get lucky on the low voltage to fuel pump. It’s a shot in the dark, but it’s free.
-The egr valve can be taken out of the equation for testing purposes by blocking off the vacuum line that goes to the egr valve. There wil be a vacuum-line going from egr valve to a solenoid, then a vac line from the solenoid to the tb, if I recall correctly. You can plug the vacuum line anywhere then reconnect it. A BB in one of the rubber connectors (not the actual plastic vac line) works well. (I’ve seen this used as a permanent repair lol).
-Ground wires are important on these trucks (and all computer controlled engines). Make sure the battery ground has good contact. Then I believe there are at least 2 other grounds. One on the intake by the coolant temp sensor and I believe one from the passenger side head to the firewall. Make sure they make good contact and haven’t been disturbed or wires broken. Especially the one near the coolant temp sensor.

If none of this helps, you’ll have to let the real mechanics chime in. I’m just a shade tree hack who’s owned 4 tbi 350’s :grin:

When i got it it had a blown head gasket. One of the heads were cracked so i ended up getting another one. i replaced all the gaskets that i could get to.

re code 54, fuel pump low voltage. from what I see the fuel pump power supply starts at the battery, goes through the fuel pump relay, and to the fuel pump. the computer monitors the fuel pump voltage, b/c it has to compensate the injector pulse time if the fuel pump voltage changes. But it should always be close to battery voltage. Somehow the oil pressure switch is involved in all this, but I don’t see how that all works. Checking the fuel pump voltage and oil pressure switch “on” resistance makes sense though.

note: info above edited

re map sensor code. One check you could make, the output of the map sensor should be about 1.25 volts at warm idle, and 4.25 volts at wide open throttle. The output in the light green wire, middle of the three wires going to the sensor.

re: shift solenoid code 82. that appears to be for the 1-2 shift solenoid you might try manually shifting using the gear shift lever from 1 to 2 and back again, does it seem to be shifting then? there’s a switch on the computer that turns the solenoid on and off, and that’s what’s being tested and not passing, creating a code. could be the computer, shift solenoid coil, or could be a wiring problem. i’d guess it was the shift solenoid coil.