Chevy TBI rough idle - but smooths out with MAP unplugged?

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Helping a friend get a 92 Blazer up and running - we got it to start but CEL was on and ran rich. It idled nice and strong. Jumpered the ALDL, got a code 33. Before I went on a parts cannon approach, I checked the MAP sensor - it was plugged in but the vacuum line was somehow disconnected. Reconnected the line. Truck isn’t running rich, CEL is off but the idle is rough.


The truck has new wires, cap/rotor, plugs. New fuel filter too - Ecogard from Amazon. Sprayed the vacuum lines, TBI to manifold flange and all along the intake for vacuum leaks. None. I stumbled across this post:

Went to the parts store, grabbed a fuel pressure test kit, I have 9psi of fuel.

Now, this truck was sitting for a while. I measured fuel pressure before the filter. A part of me wants to check the injectors before I condemn the fuel pump but there’s also the possibility CA’s ethanol-blended has simply phase-separated, water dropped out and rusted the fuel tank which also clogs the filter sock with rust. What should I check for next?
 
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Mine threw no codes but the valve itself as weak and caused tip in and other stumbling problems. Keeping it disconnected and plugged fixed the issue but now throws a code.
 
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but there’s also the possibility CA’s ethanol-blended has simply phase-separated, water dropped out and rusted the fuel tank which also clogs the filter sock with rust. What should I check for next?

Cut the fuel filter open and see if there's a bunch of rust on the dirty side of the filter.

The fuel tank in my 1984 Cavalier (also TBI) was quite badly rusted perhaps due to sitting for 10 years without being driven (the fuel pump had to be replaced to get it running again--was open circuit).

The filter sock was not clogged. The fuel filter however was a different story. It had a LOT of rust in it.

In the end I replaced the fuel filter and the fuel tank.

It idled just fine, but would bog down upon acceleration sometimes. Replacing the fuel filter and the tank fixed that problem.
 

LDM

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Check the ECM coolant temp sensor as well, not the one in the head that reports to the gauge on the dash. If it fails it can cause rough idle due to incorrect fueling from the ECM due to the sensor giving incorrect data. Had this problem with my 95 Tahoe 350 at one point and the failed coolant sensor ended up being the issue. Stock fuel pressure should be around 9-13 PSI for GM TBI. EGR can also cause spark advance issues if it is clogged and not opening/closing when it is suppose to.
 
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Another thing I remembered from the 1984 Cavalier... these TBI injectors are known to stick if they've been sitting for a long time.

I don't know if that 92 Blazer would even run with one of the injectors stuck and not working but if it did the idle would certainly be very rough!

Apply a 9V battery to the injector. It should make an audible click. If not, tap it with a wrench or a screwdriver, then apply a 9V battery again and see if it clicks.

I had to do that on the Cavalier. I was hoping that would fix the no-start but as mentioned previously, the fuel pump was dead.
 
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nthach

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Another thing I remembered from the 1984 Cavalier... these TBI injectors are known to stick if they've been sitting for a long time.

I don't know if that 92 Blazer would even run with one of the injectors stuck and not working but if it did the idle would certainly be very rough!

Apply a 9V battery to the injector. It should make an audible click. If not, tap it with a wrench or a screwdriver, then apply a 9V battery again and see if it clicks.

I had to do that on the Cavalier. I was hoping that would fix the no-start but as mentioned previously, the fuel pump was dead.
The injectors are spraying - I had a thought of sending them into Trav but these TBI injectors aren’t common for shops to dunk into a ultrasonic bath, replace the filter baskets/pintle caps and flow test.
 
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If you already changed the fuel filter, it's time to seriously looking at the fuel pump, maybe its wiring too. Not too many things are going to cause 9psi of fuel pressure and injectors would be on the bottom of my list.
 
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According to what I read, 9PSI is on the low side of what's considered normal. These systems do not run as much fuel pressure as port fuel injection does.
 
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wlk

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Definitely check the ecm coolant sensor- believe it is the one down the block under the spark plugs on the driver's side. If it had been running rich, have you ran any fuel injector cleaner threw it? Maybe plugs and injectors are a little dirty now. Also what plugs are you running in it? I have found the AC Delco copper to perform the best in these engines. Also check the egr, it could be stuck open and causing a vacuum leak. I would do the egr last as they are a pain to get to on the passenger side and has vacuum lines around it that like to break. Good luck.
 

LDM

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If you already changed the fuel filter, it's time to seriously looking at the fuel pump, maybe its wiring too. Not too many things are going to cause 9psi of fuel pressure and injectors would be on the bottom of my list.
GM TBI standard is 9-13 PSI. While its on the low end, it is still within spec. I doubt the fuel pump or injectors are the issue at this point, it is most likely something else. As the cap/rotor/plugs/wires have also been done, the most likely culprits at this point are the ECM coolant sensor, a stuck EGR, or a vacuum line somewhere that has cracked and has been missed.
 
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LDM

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Definitely check the ecm coolant sensor- believe it is the one down the block under the spark plugs on the driver's side. If it had been running rich, have you ran any fuel injector cleaner threw it? Maybe plugs and injectors are a little dirty now. Also what plugs are you running in it? I have found the AC Delco copper to perform the best in these engines. Also check the egr, it could be stuck open and causing a vacuum leak. I would do the egr last as they are a pain to get to on the passenger side and has vacuum lines around it that like to break. Good luck.
The one in the block under the head is the gauge sensor, not the ECM sensor. The ECM sensor is in the intake manifold near the thermostat.
 
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nthach

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Also what plugs are you running in it? I have found the AC Delco copper to perform the best in these engines. Also check the egr, it could be stuck open and causing a vacuum leak. I would do the egr last as they are a pain to get to on the passenger side and has vacuum lines around it that like to break. Good luck.
I threw on ACDelco GM OE cap/rotor, ACD “Professional” - now Silver wires and NGK G-Power single-platinum plugs. In the past, I would have stuck to the old “rules” for American cars - GM gets Delco, Ford gets Motorcraft or Autolite and Champion for Mopars but it seems like Delco plugs are made by Champion or Denso these days since AC Spark Plug, the AC in ACDelco is no more.

I’ll check the ECT sensor(not the single-pin head-mounted gauge sender) next time I’m at the truck.
 
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