tbi 350 idleing rough and cutting out under load - Pirate4x4.Com : 4x4 and Off-Road Forum
 
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Old 08-26-2009, 08:10 PM   #1 (permalink)
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tbi 350 idleing rough and cutting out under load

i have a tbi 350 out of a 90 suburban in my jeep. all original harness came out of the suburban and straight into my buggy. it idles rough and acts like its running really rich, then when you take it out to drive it, as soon as you step on it to put a load on, it cuts out and backfires. any ideas?
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Old 08-26-2009, 08:17 PM   #2 (permalink)
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Sounds like the timing is off or you put the plugs on wrong.
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Old 08-26-2009, 08:41 PM   #3 (permalink)
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my 91 chevy had a similar problem it would run fine when it was cold but soon as hit about 160 it would start missing popping stalling and run very rich after about 2 days of working on it I found the coolant temp sensor up top by the t-stat housing was bad about a $15 part.
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Old 08-27-2009, 07:07 AM   #4 (permalink)
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Got the MAP sensor hooked up and working? Did the engine run for you in the Sub?

Are you able to pull any codes?
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Old 08-27-2009, 08:00 AM   #5 (permalink)
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I just fought this demon with my 93 Blazer. Turns out there are two very common and easily correctable problems with tbi motors. First, the fuel pumps are extremely picky and prone to fail if run out of fuel. Sometimes it is no big thing, sometimes pressure/volume problems arise. Easy to replace, any junkyard with chevy/gmc trucks has a dozen of them.
Easier yet, the common problem is the distributor pickup, located under the distributor cap. These little pitas can cause all kinds of hell. When they start to go bad, stumbling under load occurs. When they finally give up, you will have ZERO injector pulse or power to the coil. Any parts store carries these, about 35-45$$, I carry a spare now... BTW, when installing a new pickup, lightly coat the underside with dielectric grease, you will see where the factory did this. Heat is the number one killer on these, and a swap into a smaller engine compartment has me suspicious.
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Old 08-27-2009, 05:45 PM   #6 (permalink)
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some more info that i left out, the suburban it came out of was a good running daily driver. i don't have any of the gage cluster, so no guages. not sure if this effects the ecu or signals to anything. also it has an aftermarket fuel pump that i know is putting out more than enough pressure. put new plugs in it, tried to take off the distributor but i don't have a socket small enough to take the bolts out(holding off on that for now). does the temp sensor on the front on of the motor by the t stat housing have to go to a guage cluster in order to send the right signal to the ecu? i was going to pull the sensor so i could put in a mechanical guage, but it wouldn't start when i disconnected it. also is there an idle air adjustment anywhere? its a little overwhelming not knowing where to start.
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Old 08-27-2009, 11:05 PM   #7 (permalink)
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i just remembered too, no thermostat. not sure if that makes a difference either. thins is my first tbi motor, i don't know anything about how they work. ttt

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Old 08-28-2009, 05:12 PM   #8 (permalink)
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btt
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Old 08-28-2009, 09:32 PM   #9 (permalink)
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Honestly, it appears you want to do the work and make this swap work yourself.
I'd suggest you buy the applicable "c/k truck driveability, emissions, and electrical diagnosis manual" for your engine, from GM. It will tell you every thing you need to know; All electrical diagrams, and detailed analyses of all sensors, etc. In my opinion, a "must", when you swap in an engine.
With fuel injection, you must insure the fuel pressure is to specs. The ECM/PCM counts on it being correct. The ECM can only make minor adjustments to accommodate fuel pressure variations.
The temp sensor next to the thermostat housing connects directly to the ECM-- it doesn't go to the dash instruments. The temp gauge gets its signal from a separate sensor located on the driver's side, front, just above the exhaust manifold; attach a mechanical/ or substitute temp gauge there.
The ECM relies on the temp sensor to adjust fuel mixture to accommodate engine temp; enriches a cold engine, and leans out a hot engine--the electronic equivalent to the old "choke".
The only adjustment for "idle air" is a screw in the front side of the throttle body. It's intent is to set "minimum air". It is not intended to adjust idle speed, per-say. The ECM sets idle speed by driving the "air idle control" solenoid in and out, as required. You should not have to full with the minimum air adjustment, unless you've screwed around with the throttle bore or "butterfly". There is a specific procedure for checking and adjusting minimum air. See the manual referenced above.
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Old 08-30-2009, 12:28 AM   #10 (permalink)
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thanks for all the info guys. got around to checking it, found that the vacuum line that comes from the back of the tbi that is supposed to connect to the map sensor was disconnected and had been plugged off at the tbi. hooked it back up, and it idles great now! now when i put it in gear, it runs good for a few seconds, then starts surging and gets worse and worse til it eventually kills the motor. if i take it out and try to drive it, all i get is popping and backfireing. should i plug off line to the egr diaphram and see if that does anything? any other suggestions?
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Old 08-30-2009, 12:36 PM   #11 (permalink)
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Did you mess with the Electronic Spark Control wire? The one that would've been inside the cab on a factory truck/burb. You normally unhook the wire from its connector to set timing without the ecu interfering, then hook it back up to let the ecu take back over. I believe its brown/black. If you forgot to hook it back up, then it would be staying at your timing that you set it at. I know the ecu can advance the timing up to around 35*, so that could make it run like crap, if the ecu can't control it.
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Old 08-30-2009, 08:10 PM   #12 (permalink)
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hooked a scanner up to it today, no codes! don't have a clue what it could be now. talked to a buddy and he said probably a burned valve or broken valve spring.
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Old 08-30-2009, 08:45 PM   #13 (permalink)
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Did you check for the spark control wire being unplugged liked I mentioned?
I don't think it throws a code for that, since its a procedure for setting up timing.
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Old 08-30-2009, 10:19 PM   #14 (permalink)
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no, where would i find it at? there is a white wire that comes out of the coil that isn't attached to anything. i'm not familiar with these motors at all. thanks for the info.
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Old 08-31-2009, 12:36 AM   #15 (permalink)
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It should be the tan/black wire going from the ignition module in the dizzy all the way to the ecu. It should have a connector somewhere inline to unplug the ecu control wire from the ignition module. It should be plugged in unless you are adjusting timing.
Look dead center in the picture. There should be one white wire going to the ecu, and the other white one would've been a tachometer signal wire, so it can be left unhooked.
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Old 08-31-2009, 03:43 PM   #16 (permalink)
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awsome. thanks for the info. just got home from work, going to check it out.
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Old 08-31-2009, 07:40 PM   #17 (permalink)
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Hi I dont mean to steal this thread but Im having a very similar problem on my 89 3/4 ton sub with a 350 tbi and auto trans all stock only has 57,000 original miles on it. It starts everytime and runs great when its cold but when it warms up it idles rough but never cuts out or backfires on take off that is pretty normal. and when I really romp on the gas it has a foul smell from the exhaust pipe. any help much appreciated.
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Old 08-31-2009, 09:57 PM   #18 (permalink)
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Quote:
Originally Posted by tjpolzin View Post
Hi I dont mean to steal this thread but Im having a very similar problem on my 89 3/4 ton sub with a 350 tbi and auto trans all stock only has 57,000 original miles on it. It starts everytime and runs great when its cold but when it warms up it idles rough but never cuts out or backfires on take off that is pretty normal. and when I really romp on the gas it has a foul smell from the exhaust pipe. any help much appreciated.
sounds like a bad cat.

i got around to checking the timing today, it was good. i'm starting to think its something in top end like my buddy suggested.
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Old 09-02-2009, 04:52 AM   #19 (permalink)
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Couple of questions:

Did you hook up the oxygen sensor? Look to see that the O2 sensor is switching from rich to lean in the scan tool. You can also measure this with a multimeter. Should go from 0 - 1 mV and oscillate somewhere in that range with engine running steady state.

What are the vacuum readings from the engine? Hook up a manual gauge or read the map sensor (from the scan tool) when under load, idle etc.
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Old 09-04-2009, 06:00 PM   #20 (permalink)
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The timing/esc wire on a 90 Suburban would be in the area of the power brake booster. It may be tan/black or tan/white. It is a single wire and has a black connector on it.
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