93 Blazer hunting idle and misfire

[Massey]

Bear with me here, it’s long I know but I have done quite a bit so far, and I wanted to be thorough on what’s going on and what’s been done.

So here is the scenario. 1993 Blazer 350 TBI. New engine about 2 years ago. Edelbrock intake, OEM TBI module, just rebuilt with new injectors, new Idle air control valve, new NTK O2 sensor. Cap and rotor look good, wires are new, plugs are Iridium 2 years old about 10,000 miles on them.

when we first put the engine in the truck everything was perfect. In the last couple months the truck gets a hunting idle and a misfire. It also shudders and lags a bit when accelerating.

So when I put my scan tool to it I got the LTFT at 138, sort term was the same, O2 was Bosch and pegged lean no switching. MAP was 29.8 inches (not sure if this is what it’s supposed to be) and at the time the injectors were not making a good spray, and had drips. With the IAC unplugged the misfire went away but the idle went up to 1500, and the O2 was still pegged lean.

Replaced the O2 with a NTK brand and now there is switching, but it’s 4 or 5 seconds lean, the 1 second rich, then back to lean, rinse and repeat. We also replaced the IAC valve as I have had this issue in the past from a slow worn out IAC wasn’t doing what it needs fast enough. The new IAC did improve the hunting idle a bit but now it still does it, just not as high of an rpm.

This all also starts once it’s in closed loop mode. In open loop there is no hunting, no miss. I’m thinking it’s a lean miss as there is virtually no miss when the engine is cold.

TPS shows a good steady voltage and percentage all the way up and down its range, and is at 0% (.48v) no throttle and 100%(4.5v) at full throttle.

so right now I’m leaning towards low fuel pressure but since it starts fine, and runs fine in open loop I’m not sure on this one. There is no schrader valve to test from and I do not have access to the correct test fittings. Also an EGR leak comes to mind but again why doesn’t it cause issues in open loop. Unplugging the electrical or vacuum hoses makes no difference. No known vacuum leaks, or at least none detectable with starting fluid at seams. No signs of exhaust leaks. Grounds all appear good, and test out with my meter just fine. The one reading I’m at a loss on is the MAP sensor, it doesn’t change much and likes being at 29.8 mg.

any advise would be great.

 

[Schurkey]

ANY misfire, for ANY reason on the driver's side (left) bank will cause the O2 sensor to read lean.

Find out why the MAP sensor is non-responsive. (Disconnected hose? Perforated/torn/leaky hose? Failed sensor? Defective wire harness?)

Verify the fuel pressure. Pressure gauges can be gotten from many different parts stores under their "loaner-tool" program. You might have to buy a TBI adapter. Make sure the adapter has the same Schrader valve thread size as the pressure gauge you have access to.

I'd be looking at the coolant temp sensor, too. That's a major player for determining how much fuel to inject.

 

[Massey]

I did forget to mention that the CTS is reading right on, I doubled checked coolant temps with the reading on my scan tool. I have found some diagnostic information on the MAP and the reading being 29.8 in hg could be an open circuit or bad sensor. When the vacuum goes up the voltage goes down, so if the pcm reads 0v then logic says start looking there. I will see the truck this weekend, so if there are any other ideas please share.

 

[Schurkey]

the reading being 29.8 in hg could be an open circuit or bad sensor. When the vacuum goes up the voltage goes down, so if the pcm reads 0v then logic says start looking there.

Depending on your altitude and weather, that sensor is reading NO vacuum. First thing to check is to see if the hose is disconnected.

 

[Erik the Awful]

It sounds more like a vacuum leak than low fuel pressure, but definitely check your fuel pressure.

Fix those dribbling injectors!

Try putting a vice grip on the hose to the brake booster. That's a potential vacuum leak that's hard to find with a spray can.

 

[Massey]

Injector dribble was fixed.

I do not live at a high enough altitude to not read any vacuum. Sea level describes where I live the best.

will try the booster, it is still the factory unit on the truck.

 

[Schurkey]

I do not live at a high enough altitude to not read any vacuum. Sea level describes where I live the best.

MAP--Manifold Absolute Pressure sensor. That's a pressure sensor, not a vacuum sensor. Big difference, since at least us old guys are used to thinking in terms of the reading on a VACUUM GAUGE.. The sensor is calibrated from "0" pressure (total vacuum) to "Full" pressure, (no vacuum) about 30 inches of mercury. An idling engine might have 13 inches of mercury pressure--or 17 inches of vacuum. See the difference?

You're reading damn near 30 inches of mercury pressure. Almost "full" atmospheric pressure. That MAP sensor thinks the throttle is FULLY OPEN. NO vacuum. The engine should be commanded rich. Horrible fuel economy. If you had a vacuum leak destroying vacuum, the additional air entering the manifold would raise your idle speed.

The most-likely cause is a disconnected vacuum hose from the vacuum source to the sensor nipple. The sensor isn't connected to vacuum, therefore it's not sensing any.

A plugged vacuum port, a leaky hose, a failed sensor, or a broken wire harness are other possibilities.

 

[Massey]

The engine is commanding full fuel available with the LTFT being pegged to 138 which is max for this engine. Even at 138 fuel trims, the O2 still remains lean more than rich.

On the document I read on the function of the MAP for this truck, the voltage decreases as vacuum increases. So atmospheric pressure will yield a 4.7 volt reading to the computer, 5in Hg is 3.9v, 10in Hg is 3.0v and 20 in Hg is 1.1v.

Yes 30 in hg is max vacuum, but it is also 0v reading to the computer, which my issue could be electrical. I wish I had the truck here to look at but it is only here on weekends. I do know that there are no broken vacuum hoses, and that all vacuum connections are healthy. I inspected all that last weekend when it was here. I also verified that all connections were to the proper ports according to the diagram on the core support.

when I have access to the truck again I’m going to test that the reading at the sensor is the same as the reading at the computer. I’m also going to check for a vacuum leak at the booster.

 

[Schurkey]

The engine is commanding full fuel available with the LTFT being pegged to 138 which is max for this engine.

Yeah, I'd expect that, until you get the MAP and O2 straightened-out. Both sensors are telling the computer to add fuel--heavy load, lean mixture.

Even at 138 fuel trims, the O2 still remains lean more than rich.

IF (big IF) the fuel pressure is good, and the injector(s) aren't plugged, I'd put that down to misfire.

On the document I read on the function of the MAP for this truck, the voltage decreases as vacuum increases. So atmospheric pressure will yield a 4.7 volt reading to the computer, 5in Hg is 3.9v, 10in Hg is 3.0v and 20 in Hg is 1.1v.

Yes, voltage decreases as vacuum increases. Or, voltage increases with pressure.

Yes 30 in hg is max vacuum, but it is also 0v reading to the computer, which my issue could be electrical.

This doesn't make sense.

The MAP sensor is showing atmospheric pressure, so the voltage to the computer MUST be high--not "0v".

Again, "30 in hg" is strong vacuum only IF (big IF) you're reading a vacuum gauge. You're not. You're getting a reading from a pressure sensor, which means that "30 in hg"--or 29.8 in your case--is NO VACUUM (atmospheric pressure.)

I don't get how the computer can show 29.8 in Hg on your scan tool, and report "0" MAP voltage at the same time--they're opposite ends of the scale.

I threw my Red Brick on my '88. Key on, engine off (no manifold vacuum) I'm showing 29.6 in Hg; something like 4.7 volts. Start engine, warm idle, I have 7.9--8.1 in Hg, and I think 0.8 volts.

I wish I had the truck here to look at but it is only here on weekends. I do know that there are no broken vacuum hoses, and that all vacuum connections are healthy. I inspected all that last weekend when it was here. I also verified that all connections were to the proper ports according to the diagram on the core support.

when I have access to the truck again I’m going to test that the reading at the sensor is the same as the reading at the computer. I’m also going to check for a vacuum leak at the booster.

I'd like to see what the MAP sensor is reading with the key on, engine OFF. Then start the engine--there should be a dramatic change in the reading, from high voltage/high pressure, to mid/low voltage, mid/low pressure.

 

[Massey]

My buddy is supposed to bring the truck over tomorrow (today by the time you read this) and I will see. I don’t remember my scan tool showing MAP voltage, but it may just be on a different screen. My LTFT and STFT on this truck are 4 pages apart... that is really convenient for troubleshooting

the O2 sensor is new NTK, and it does switch, just not much. The Bosch that was in it, was 2 years old and stuck lean.

I’m thinking you have the properties of measuring the vacuum wrong. A vacuum is measured in inches of Mercury, the more vacuum the more in hg that is on the scale. Not the greater the atmosphere the greater the in hg. If there is no vacuum there is a reading of 0in hg. That’s full atmospheric pressure. And pressure is measured in psi, or bar.

here is a link to a website that explains the testing of the MAP sensor

https://troubleshootmyvehicle.com/gm/2.2L/how-to-test-the-map-sensor-1