Loss of power 99 Bravada

[YooperJ]

HI,
Well here is my problem. Back about 6 months or so I had a problem when I first started the car. When moved forward it would hesitate badly until it cleared out and then ran fine. It was like when the float was leaking on a carb, after first start it would run bad until the fuel was all burned up from the fuel flooding the engine. I had no check engine lights and plugs were just changed 10K previous. I had to change the oil lines so I also put a new coolant temp sensor in and did a coolant change at the same time. Local parts store guy told me they sell lots of them and sounds like it could fix the problem. I didn't think so but since I needed the coolant change, what the heck. The problem has continued but would clear up and run fine after the first minute, just as before.

The other day I changed the shocks and decided to do the CMFI upgrade. The plenum was really full of carbon (115K) as was everything else in there. Cleaned it all out using carb cleaner and rags. Throttle body throttle plate was really bad also. Installed a new Standard brand Fuel injector and gaskets, all went fairly well.

Started it up and no fuel leaks but ran rough because of the carb cleaner. The initial problem is still kinda with me. Now, instead of just hesitating, it misfires a couple of times and then everything is just fine. Again, this happens ONLY on first start-up of the engine and not again until it is shut off and restarted and then only sometimes. The engine seems to have a lot more pep now but this hesitating and missing is driving me nuts. No engine codes or service lights. No vacuum leaks that I can find, everything seems fine other than start-up.

Any ideas????

[Chris Stewart]

Sounds lean to me too. It might be low fuel pressure. Fuel pressure test kits are available at most chain type auto parts stores....look for 60 to 66 psi when you turn the keyswitch on.
Here's where you connect the gauge,

[YooperJ]

Thanks for the response Chris! Checked the Fuel Pump pressure; Key on - Engine off = 62 psi then drops to 56 psi. Fuel Pump seems to be within specs. After about 45 mins the pressure dropped to 50 psi. Any other ideas or help will always be appreciated. That's what makes these forums so great!!!

[Chris Stewart]

Because we have professional mechanics checking here every now and then, let's get them a fuel pressure at idle and what the fuel pressure does when you open the throttle for a half second.

[YooperJ]

Unhooked the battery for an hour. Hooked it back up and took it for a 60 mile ride. After the initial start and missing for 30 secs it ran great. Mileage computer said averaged 21.2 mpg which is pretty good. Lots of power, more than before the injector change. I shut it off a couple of times and restarted but had the same hesitation and coughing for about 30 secs and then fine.

Checked Fuel pump pressure again, Key on-Engine off = 62 psi then lowers to about 56 or so. Engine idling = 55 psi and when I accelerate it pressure jumps to 62 and then back down. At 1000, 1500, 2000 & 2500 RPM the pressure remains at 53 psi.

Vacuum is 19 in at idle and at 1500-2000 RPM it goes to 20 in. When I accelerate it goes to 23 in and then back to 19 in.

[Gabe25]

Here is a thought. Being that you cleaned all the carbon out of the Plenum. Did you remove the EGR Valve and check for carbon chunks. Your trouble sounds like you mite have some chucks floating around and resricking the pindle to open and close properly. There are gaskets out there that have a screen that prevents the chucks from getting in the EGR valve. Let us know how your doing.

[YooperJ]

I cleaned all the valves I could, EGR, IAC, PVC. Still have the problem.

[old_master]

Your fuel pressure and leakdown rate is fine. Your 30 second time frame could be a clue though. Each time the engine is started, the PCM controls fuel mixture in "open loop operation", which means for the first 30 seconds, (sometimes longer), the PCM ignores oxygen sensor data. It relies on data from other sensors to adjust fuel mixture until the oxygen sensors are up to their normal operating temperature. When they are heated up, the PCM switches from open loop to closed loop operation and then uses the oxygen sensor data to adjust fuel mixture. The coolant temp sensor and the inlet air temp sensor must be within one or two degrees of each other when the engine is cold. That might be what your parts guy was thinking the problem was. It might very well be just that, only it might be that the inlet air temp sensor is off. Without a scan tool to dial in to each circuit and watch it, it's hard to say exactly what's causing the problem. Throwing money at it can get expensive, so it might be worth while to have a shop check it out for you... unless you have access to a scan tool and post the data here for us to analyze.

[YooperJ]

I do have access to an OTC with 01 deuce cartridge. I check and post back. The coolant sensor is new and it did the same with the old. I was also thinking about the open/closed loop but have difficulty using the Deuce software. The Pathfinder one was more familiar to me. Anybody know where I can get the user manual for the Deuce software by OTC?

[old_master]

I understand that the coolant sensor made no difference, but it could be the IAT (inlet air temp) sensor that's not reading correctly, that's where the scan tool will help. Try a google search for the OTC and you might find a downloadable owners manual in a pdf.

[YooperJ]

The IAT started out at 80 F and went to 115 F. After running the engine for a bit I shut it off and waited a couple of minutes and started it back up, same miss problem, the IAT was 118 F start and then went to 127 F. I hav access to a2000 Bravada and will check what that one does, it runs fine. The only other thing with my scan tool I found that B1S1 (STFT Bank 1 Sensor 1) changed value from a -3 to 9 with engine running, engine off key on =0 . B1S3 (STFT Bank 1 Sensor 3) is at a fixed 99.2 with engine off key on or in a running state.

[old_master]

You need to compare the IAT and ECT after the engine has set over night. Both sensors must be stabilized at ambient temperature, do not start the engine when comparing readings.

Oxygen sensor output should range from zero to .9 If they indicate negative voltage or anything above .9 there's a problem with the sensor. The bank 1 sensor 3 is located after the converter. When the engine is in closed loop, around 2,000 RPM, the sensor should indicate approximately .4 or less and remain steady. Bank 1 sensor 1 and bank 2 sensor 1 should vary between approximately .2 and .7 The PCM strives for a reading of .45 which calculates to a fuel mixture of 14.7:1, the ideal mixture.

[YooperJ]

Checked the ECT 84 F and IAT 86 F when cold before startup. The ECT increased but the IAT held pretty steady at 86 degrees. Will check the other things tomorrow and also compare it to the 2000 Bravada.

[old_master]

If your ambient temperature was within a couple of degrees + or - of 85F, the ECT and IAT voltages are fine. The ECT will increase as the engine warms up, the IAT will increase slightly as underhood temps increase. After the engine is at operating temperature, (195F) the IAT should be slightly higher than ambient temperature.

Check those oxygen sensor voltages again, something's screwy there.

[YooperJ]

Did some tests and these are the results at 2000 RPMs and idle while in Closed Loop.

2000 RPMs
O1S1 - .06 - .85
O2S1 - .07 - .84
O1S3 - .31 - .85

B1S1 - 1.5 - 3.9
B2S1 - 1.5 - 3.9
B1S3 - 99.2 steady


IDLE
O1S1 - .05 - .9
O2S1 - .05 - .9
O1S3 - .31 - .85

B1S1 - (-3) - .78
B2S1 - (-3) - .78
B1S3 - 99.2 steady