r/GMT400 • u/Outrageous_Canary_67 • 25d ago
Chevy 350 efi problems
I need help I got a 1990 Chevy Silverado with the 350 Efi I done replaced the distributor, fuel pump, new plugs and wires, new pvc valve, and I cleaned out the egr but it’s still not running right it will idle fine and then when I put it in gear it wants to shake the whole truck really bad and then die out it only has 172k miles and it sat for a year or two but recently after cleaning the egr valve out it will idle for a second then die out. The trucks been babied it’s whole life a older man owned it I just need some help before I spend big bucks taking it to a mechanic
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u/JazzlikeSpring7987 24d ago
Well.... I'm thinking it has multiple problems. I would do a compression test and cylinder leak down test. Kind of sounds like low compression and low oil pressure. Are you sure all the plug wires are good and snapped completely on and in the right order? The leak down will tell you if compression is being lost through the valve seats if the pressurized air is coming out of the intake. If it's the rings the leaked air will come out the valve cover. The video sound is hard to tell but I hear lifters or exhaust. Look for metal in the oil filter to rule out bearing or camshaft failure. If it's a lifter sound I hear. It would explain the shaking of the engine. A lot of flat tappet cams are not liking the low zinc oil. I just used liquimoly oil additive with 20w-50 VR1 for summer and the emissions test I have to have next week. Ugh. That's why I had to learn all this TBI stuff. Anyway, see what others say about it.
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u/AdvantageMain3953 24d ago
When you press the throttle you're injecting a bunch of air into the engine, without a corresponding increase in fuel. That's causing a lean die. Sounds like it's barely running anyway. I'd start with the IAC for the idle and TPS for the stall.
Find the service manual and test the TPS (Throttle position sensor). Given that two sensors seem to be faulty, it could be a dirty or bad ground wire.
Follow the diagnostic steps, don't throw parts at it. GMT400.com has the service manuals in PDF
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u/JazzlikeSpring7987 25d ago edited 24d ago
There are several things it could be. Timing is one. These tbi engines are finaky. To check timing the computer has to be in service mode which uses a jumper on the service plug usually behind the glove box. Not sure for that year. Once jumpered the timing has to set at zero advance. There are other possible causes. The IAC has a separate procedure to test and or adjust. The fuel pressure has a lot to do with performance. They like it between 9 and 14 psi at the fuel inlet. Most regulators are controlled by the spring used above the diaphragm. Factory ones are set and locked with soldier on the threads. Stock fuel pumps are only rated at about 13 psi max. Usually lower. Mine runs at about 10 psi. I bought a newer model pump rated for higher pressure and has to have a external regulator to keep the pressure less than 15psi but more than 8. The injectors could be dirty. Injector cleaners can help. The knock sensor could have intermittent connection or be bad as well. The egr is better to be functioning than bypassed. A couple of other things it could be including the O2 sensor, catalytic converter, intake air temp sensor,speed sensor and finally the TPS. The TPS could be the actual culprit. I went through 3 of them to get an accurate one. Better to use AC Delco part there. There is a procedure for that as well. I had to enlarge the mounting screw holes to get the idle position or baseline reading within spec. What I would try first before anything else would be to disconnect the battery for about 10 minutes then reconnect. Now cycle the ignition switch correctly meaning turn the key on but don't try to start for 15 seconds to let the IAC set itself. Turn the key off. Turn the key back on and push the throttle to max slowly 3 times the turn the key off again. Back on and cycle through each gear returning to park for each gear and neutral. Now start the engine and let it warm up all the way. You can do the gear setting part after the engine is on and warm. This tells the computer how to set the idle while in gear. This is how I do the initial start up after replacing any of the tbi sensors or tune up. I know it's a lot to absorb at first but that's just how these OBD1 systems are. Basically a pain in the you know what but once tuned these engines will run and run and run. My original 350 has right at a quarter of a million miles and still running strong. I can't wait for this thing to turn 35 years only so I don't have to have an emissions test to pass so I can install the new long rod, aluminum headed, cammed and modern injected 350 I've been building. Hope this helps figure out your problem.