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1993 Sierra 350 TBI running rough

OleGreen93

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OleGreen93 New Member

OleGreen93

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Hi everyone new to the forums here hoping i can find some suggestions on what is wrong with my truck.

Some information on the truck:
1993 K1500 Sierra 350 TBI with 174k miles
Engine has been rebuilt with around 35k on the rebuild. It has an RV cam and high volume oil pump. Other than that it is stock.

I just received the truck from my dad and im trying to diagnose some issues it has had for some time. When turning the key to the "on" position the fuel pump primes for around 10 seconds before shutting off. Is this normal? I remember years ago when i used to drive the truck the pump would only prime for 2-3 seconds before shutting off.

After the 10 seconds of fuel pump priming, it starts right up but immediately runs very rough where the engine, tailpipes and doors shake if open. The exhaust sort of flutters out the tailpipes and smells very much like un-burnt gas or as if it is running rich. I have so far changed the spark plugs, map sensor, o2 sensor, fuel filter, egr valve, fuel regulator, both temp sensors, thermostat and cleaned the IAC with no improvement. I checked for any vacuum leaks or bad vacuum lines and can't find any. The truck will continue to run rough even when warm and at a stop light the idle will drop to almost wanting to stall but then will surge back up and then drop to almost stalling and so on until i start driving again. I have to hold the brake and give it gas to about 1000 rpm so it wont have the surging idle. It has a 195 degree thermostat but seems to never reach full operating temp. From the gauge cluster it looks like it stops at around 175 degrees or so.

I will be checking the ignition timing this afternoon to verify that it is correct and plan to test the fuel pressure as well. Do you guys have any ideas what might be the problem?

Thank you for any assistance and i'll post back with my ignition timing findings.

Mikebtte Enthusiast

Mikebtte

Posted
Posted

Fuel pump will turn on for about 10 seconds when you first turn them on followed by shutting off until you start the truck.

Ignition timing should be set @ 0 while at idle and at temp. Unplug the brown wire and the P/S firewall when checking timing.

First thing I would check would be fuel pressure. Always a good place to start on any TBI job. Pressure regulator could cause a high or low fuel pressure issue.

Second would be to get a ALDL cable and monitor everything. The cable would be cheaper than tossing parts at it, but you dont have many parts left to throw. I assume the ECM was tuned for the new cam. 

Usually Temp sensor, MAP would resolve the issue if you had one. Also make sure your EGR valve is closed at idle, they can stick open.

OleGreen93 New Member

OleGreen93

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4 hours ago, Mikebtte said:

Fuel pump will turn on for about 10 seconds when you first turn them on followed by shutting off until you start the truck.

Ignition timing should be set @ 0 while at idle and at temp. Unplug the brown wire and the P/S firewall when checking timing.

First thing I would check would be fuel pressure. Always a good place to start on any TBI job. Pressure regulator could cause a high or low fuel pressure issue.

Second would be to get a ALDL cable and monitor everything. The cable would be cheaper than tossing parts at it, but you dont have many parts left to throw. I assume the ECM was tuned for the new cam. 

Usually Temp sensor, MAP would resolve the issue if you had one. Also make sure your EGR valve is closed at idle, they can stick open.

I just finished checking the base timing and it was spot on at 0 degrees before tdc. I rented a fuel pressure test kit but won’t be able to test until hopefully tomorrow due to rain. I’ll reply back with my fuel pressure results and go from there

sixteenacrewood New Member

sixteenacrewood

Posted
Posted

I have the same truck, 93, 350, c1500, 418,000 miles

 

I chased around a fuel related problem for a week, tested everything, turned out to be the spark control module in the distributor.

 

Definitely check fuel pressure first, should be between 9-13 psi

 

You should get the Haynes Repair Manual for your truck and a digital multi meter and vacuum pump, if you don't already have them.

 

To me It sounds like a bad or stuck egr valve, I saw that you replaced it.

 

I would do a methodical exam, test, of all the sensors, even new ones

OleGreen93 New Member

OleGreen93

Posted
  • Author
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On 4/19/2019 at 1:26 PM, sixteenacrewood said:

I have the same truck, 93, 350, c1500, 418,000 miles

 

I chased around a fuel related problem for a week, tested everything, turned out to be the spark control module in the distributor.

 

Definitely check fuel pressure first, should be between 9-13 psi

 

You should get the Haynes Repair Manual for your truck and a digital multi meter and vacuum pump, if you don't already have them.

 

To me It sounds like a bad or stuck egr valve, I saw that you replaced it.

 

I would do a methodical exam, test, of all the sensors, even new ones

Very good suggestions. I have not been able to test the fuel pressure yet due to weather and the holiday weekend but will hopefully be able to this afternoon. If I find out that the fuel pump is supplying the correct pressure I was concerned that it could be something internal to the distributor. When i changed the spark plugs the ones that came out had around 35k miles on them and were all black from what I thought was too much fuel or an incomplete burn. Once I test fuel pressure that should steer me in the right direction.

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