Cold Start Problem

[squeasel]

Hi. First post here. I am having trouble starting my 96 Tahoe (5.7L 4dr 4wd <120,000 miles) when cold. I must pour some fuel into the Throttle Body to start. Once warm the truck starts fine. The truck seems to run fine though a few years ago I thought I had less power when towing.

I have checked the fuel pressure. The gauge reads 55 PSI with ignition on but before start. It maintains 55 PSI after start (using fuel in the TB) and on rev up to 3000 RPM. Before changing the fuel filter the gauge registered a drop in pressure to about 52 PSI on rev up, otherwise the same.

I have read about a number of possibilities on this forum and other reference material, like Fuel Pressure Regulator, Sequential Injection (and an intriguing upgrade), and Fuel Pump (note that the filter has been recently replaced).

What should my next test be? Is 55 PSI on cold start a problem? I am the original owner and several years ago we had a problem with the distributor, but that was fixed. Otherwise the car is original.

Much thanks in advance.

 

[Ceejkay]

welcome im sure someone will chime in soon to help.. if you dont have tbi i would say spider assembly..

 

[SunlitComet]

fuel pressure seems fine I would look into fuel enrichment because it seems like you computer does not realize the engine is cold. I left a member in Cali hanging with this problem when I left. Did not have the time to figure it out. Get access to a scanner with live data and plug in with the engine really cold. See what the temp sensor reads and compare to surrounding atmosphere at same time. same with map sensor as well.

 

[squeasel]

@SunlitComet ... easily done. I have a scanner with live data feature and I will check in the morning and post results. Thanks.

 

[squeasel]

Here's the data taken when car was cold and not running, key on run:
MAP 30.1 in. Hg (manifold pressure?)
ECT 48F (engine coolant temp?)
IAT 52F (Intake Air Temp?)
MAF 0.3 lb/min (Mass Air-flow?)
TPS 0% (throttle position?)

The temp readings are probably correct. I have no idea about the MAP number but according to weather.gov the barometric pressure in the area is 30.18 inches.

Is this data meaningful? What other test should I run before random parts changing?

Much thanks!

 

[SunlitComet]

if I am reading the MAF sensor correctly it says 0.3lbs per sec and with the conversion of
1 pound = 453.59237 grams



you are running 136.077711 gram per seconds thru that maf sensor while it is not running. The service manual says this. "At idle, it should read between 5-7 gm/Sec on a fully warmed up engine." If these values are correct then it would indicate something is goofed with the maf sensor. This my best guess at this point so if you replace things randomly start with that and only use a delphi/acdelco unit. can you change the reading from english scale to metric scale just to be sure about readings.

 

[squeasel]

I double-checked the MAF ... it is 0.3 lbs/min not sec. I switched the scanner to metric and the new reading is 2.44 g/sec. However, now that I have a spec to shoot for (i.e. 5-7 g/sec) I check that out.

BTW: I forgot to note that I have no Check Engine Light (or whatever it is in a Tahoe) and the car passed Calif. Smog very recently (and after the cold start problem).

I am going to fire up the engine and see what readings I am getting and post the results.

Thanks,

---------- Post added at 11:43 AM ---------- Previous post was at 11:20 AM ----------

More data in metric: before start MAF is @ 2.44 g/sec. After starting the car (using a splash of gas in the Throttle body) the MAF is 17 g/sec going to down to a bit more than 7 g/sec after a couple of minutes but before the engine has reached normal temp. On restart, the MAF again climbed high (13+ g/sec) but quickly settled to near 7 g/sec. I imagined if I let it warm completely I would be in your acceptable range.

However, I am wondering why MAF is giving me a high number on start up. Doesn't the car start cold with a rich mixture? And why is so much air getting in since the throttle body "flap" is closed ... or is that opening too early?

Thanks.

 

[SunlitComet]

becuase at start various emissions components like egr or purge solenoids open and close effecting the airflow in addition to density of air.