Engine Missing While Cold

[95 Tahoe]

I have a 95 Tahoe with a TBI 350. I rebuilt this engine earlier this year due to pitted cylinders. Since, the rebuild I haven't been able to get it to run quite right. It runs fairly well once warmed up. However, on cold start it idles terribly. During the rebuild I used an RV cam, which was supposed to be mild enough to work with the OE chip. During cold start the engine acts as if it's flooding with a miss and vacuum is low. I fairly certain it is. Once, I apply a little throttle it goes into closed loop and clears up. Once the engine warms up and the engine goes into closed loop the vacuum increases to normal range. The IAC or ECM is a little suspect. The IAC will not zero out when the OBD port is jumped. However, I can see it working on the scanner.

Things I know.
Map Sensor -good
ECT(Coolant temp sensor) -good
EGR Sol and Valve - good
Distributor Assembly - Replaced
Plug Wires- Replaced
Base Timing- Set to 0 w/ wire unplugged
Throttle Body/Fuel Pressure regulator - Rebuilt

 

[Keeper]

closed vs open loop.... coolant temp sensor for ecm?

I sound like a broken record... lol

 

[95 Tahoe]

closed vs open loop.... coolant temp sensor for ecm?

I sound like a broken record... lol

That one is good and is displaying the right readings on the scanners.

 

[Komet]

Engine missing when cold! Where does it go?

Seriously though, you probably need a tune to compensate for the cam.

 

[95 Tahoe]

I think I'm on the right track. I think it's the new map sensor is reading wrong or a possible wiring issue. While it's running rough and flooding I plugged a mightyvac into the map sensor and dropped it to 20inhg the idle immediately stabilized. The manifold pressure climbed from 16 to 18 inhg. I looked back at some of my old logs. The reading was 55 KPA which converts to about 13.5 inhg.

 

[Schurkey]

What is actual manifold vacuum vs. vacuum at the MAP sensor? (Use a different manifold-vacuum port to test)

What does the scan tool show for MAP readings vs. actual vacuum/manifold pressure?

Was the cam degree'd, or just crammed-in "dot to dot"? What is the cranking compression?

 

[95 Tahoe]

What is actual manifold vacuum vs. vacuum at the MAP sensor? (Use a different manifold-vacuum port to test)

What does the scan tool show for MAP readings vs. actual vacuum/manifold pressure?

Was the cam degree'd, or just crammed-in "dot to dot"? What is the cranking compression?

I just went out and checked it. I'm getting 45KPA on the sensor and 16 inhg on the vac gauge this morning. This morning the math seems to work out when I covert it over. The brake booster and EGR give the same vac readings. The cam was not degree timed. I just went dot to dot. Hindisght I wish I would have degree timed it to be sure. I only checked compression on one cylinder. I had 150 PSI on #2 while cold. Today I plan to check compression on all 8 and verify TDC with a piston stop.

 

[95 Tahoe]

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I've included a screen shot of the short log this morning. Engine was cold and still in open loop.

You must be registered for see images attach

 

[Schurkey]

I just went out and checked it. I'm getting 45KPA on the sensor and 16 inhg on the vac gauge this morning. This morning the math seems to work out when I covert it over.

Your barometric pressure when taking these measurements was 29.65. Of course, it varies by weather and altitude. For ease of mathematics, I usually use 30 in/hg as a baseline. Using actual barometric pressure is more accurate.

45kPa = 13.3 in/hg manifold pressure
30 - 13.3 = 16.7 in/hg manifold vacuum.
13.3 + 16.7 = 30

With actual baro pressure, your manifold vacuum would be 16.35.

So, yeah, your numbers are valid.

The brake booster and EGR give the same vac readings.

Glad you checked a different vacuum port for the manifold vacuum reading. I wanted to verify that the MAP sensor port 'n' hose isn't restricted and giving false readings.

All seems good.

The cam was not degree timed. I just went dot to dot. Hindisght I wish I would have degree timed it to be sure.

A common lament. I degree EVERY cam I install. It's a pain in the ass, and "shouldn't" be needed. But if there's trouble later, it's something I don't have to be concerned about, or second-guess myself.

I only checked compression on one cylinder. I had 150 PSI on #2 while cold. Today I plan to check compression on all 8 and verify TDC with a piston stop.

Is 150 psi reasonable with your altitude and compression tester gauge? That'd be entirely normal around here.

Which is probably good news, because if the cam timing were off, compression pressure would--probably--be higher or lower than "normal".

Verifying TDC is always a good idea, and part of the process to degree a cam--so I always verify TDC.

I've included a screen shot of the short log this morning. Engine was cold and still in open loop.

You must be registered for see images attach

You have a wide-band O2 sensor?

Overall, I'm not seeing a reason for the poor cold idle. Is the heated air intake on the air cleaner snorkel working? Got a hose to the heat shield on the exhaust manifold?

 

[95 Tahoe]

Your barometric pressure when taking these measurements was 29.65. Of course, it varies by weather and altitude. For ease of mathematics, I usually use 30 in/hg as a baseline. Using actual barometric pressure is more accurate.

45kPa = 13.3 in/hg manifold pressure
30 - 13.3 = 16.7 in/hg manifold vacuum.
13.3 + 16.7 = 30

With actual baro pressure, your manifold vacuum would be 16.35.

So, yeah, your numbers are valid.


Glad you checked a different vacuum port for the manifold vacuum reading. I wanted to verify that the MAP sensor port 'n' hose isn't restricted and giving false readings.

All seems good.


A common lament. I degree EVERY cam I install. It's a pain in the ass, and "shouldn't" be needed. But if there's trouble later, it's something I don't have to be concerned about, or second-guess myself.


Is 150 psi reasonable with your altitude and compression tester gauge? That'd be entirely normal around here.

Which is probably good news, because if the cam timing were off, compression pressure would--probably--be higher or lower than "normal".

Verifying TDC is always a good idea, and part of the process to degree a cam--so I always verify TDC.


You have a wide-band O2 sensor?

Overall, I'm not seeing a reason for the poor cold idle. Is the heated air intake on the air cleaner snorkel working? Got a hose to the heat shield on the exhaust manifold?

I do not have wide band 02 sensor. It is just part of the file in Tuner Pro I'm using. It was a profile that was already made that works with my ECM.

I am very close to seal level. My manual said compression should be 150 but I know that can change with elevation.

Today I ignored the proper procedure of setting timing. I set the timing while watching the vacuum gauge and rolled the distributor until it gave me the highest reading which was 18 -19 inhg. It seemed to run a lot better. Even tonight when I went out and cranked it after it cooled off it didn't stumble as bad. Tomorrow, I am going to verify TDC with a piston stop to be sure. I did not go back and verify with a timing light to see where it ended up. Maybe the mark on the harmonic balancer or timing cover is incorrect. Both were replaced during the rebuild.

Second thing I am chasing is the IAC. It will not reset to closed by jumping A&B on the OBD1 connector. However, it does work to adjust idle speed while the engine is running. I am suspecting the ECM could be failing.

Last thing I want to verify is if the fuel return line or the fuel pressure regulator is a problem. I rebuilt the regulator and throttle body when I rebuilt the engine.

The heated air intake is not connected. I live in Louisiana not sure if it's needed or not.