1998 Chevy Silverado same behavior as old style accererator

Tiny
TOM NIGHTINGALE
  • MEMBER
  • 1998 CHEVROLET SILVERADO
  • V8
  • 4WD
  • AUTOMATIC
  • 14,000 MILES
Low end hesitation after engine is hot.
engines acts like it loads up during stop light stops
engine chugs along till after 1300 RPM
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Sunday, April 4th, 2010 AT 11:34 AM

5 Replies

Tiny
CARADIODOC
  • EXPERT
Hi Tom. GM actually does have enough trouble with fuel pressure regulators that it is worth mentioning. Pull the vacuum hose off. If it has fuel inside the hose, the regulator is leaking. That will add a pile of fuel that the Engine Computer isn't expecting.

The Mass Air Flow sensor has the biggest say in how much fuel is commanded by the computer. If it is dirty, it will not accurately measure all of the incoming air so too little fuel will be sprayed from the injectors.

If your engine uses a barometric pressure sensor, the computer will believe its reading as long as it is between.5 and 4.5 volts. If the value is wrong, the computer will think you're up in the mountains or below sea level and will be asking for the wrong amount of fuel. Chrysler engines use only this exact same sensor to measure barometric pressure before the engine starts, then to determine fuel metering based on engine load once it is running. GM will sometimes use this too as a MAP sensor as a backup in case of failure of the MAF sensor.

If you have a single cylinder misfire due to a loss of spark, the oxygen sensor will detect the unburned oxygen in the exhaust. The computer will react by commanding an increase in fuel delivery from the other three cylinders on that side of the engine. No matter how much fuel it adds, there will still be that unburned oxygen from the misfiring cylinder. (O2 sensors don't detect unburned fuel). If you connect a hand-held computer, called a scanner, that can read live sensor data, you will see that O2 sensor reporting a lean condition while you see black smoke or smell raw fuel from the tail pipe.

A less common problem causes the same symptoms if there is a leak in the exhaust pipe before the first O2 sensor. Between the pulses of exhaust flow, the momentum creates little pulses of vacuum. Air can be drawn in through that leak and be detected by the O2 sensor. Again, no matter how much fuel the computer adds, the O2 sensor will continue to see unburned oxygen. An important clue here is your observation that the problem doesn't start until the engine is warmed up. The oxygen sensor doesn't start to do his thing until it reaches 600 degrees. They have electric heaters built in to hurry them along, but it can still take a few minutes for them to get hot enough to go into "closed loop". Until that happens, the Engine Computer bases all of its fuel metering decisions on all of the sensors except the O2 sensor.

Caradiodoc
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Sunday, April 4th, 2010 AT 4:13 PM
Tiny
TOM NIGHTINGALE
  • MEMBER
I don't know if you saw my additional comments.
I have replaced: fuel pressure reg, Distrib, Fuel pump,
EGR Value, My scanner had EGR codes, Miss fire codes, P0404 P0405 related to EGR.
I removed the plentum and checked the FPRegulator 3-times to make sure there isn't a vac leak. I used starting fluid to check around this are also.
What is the next step to find the problem?
Cleaned the MAF already. How do you check out the MAP. If not the MAP next step.

Which vac. Hose line
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Sunday, April 4th, 2010 AT 5:38 PM
Tiny
CARADIODOC
  • EXPERT
Yup, I saw your comments. Does your scanner read live sensor data? If so, look at the MAP sensor's barometric pressure reading and compare that to the local weather report. The sensor's signal must be between.5 and 4.5 volts. Anything outside of those values will set a fault code. Still, the reading can be wrong, but if it is within those values, the computer will be happy and will believe them.

The next thing to do would be to look at the long and short term fuel trim numbers. If they are high positive, the computer is trying to add fuel beyond the pre-programmed values in an attempt to correct a lean condition. This is where a vacuum leak, misfire, or exhaust leak would be suspect. Anything that allows air to enter the intake without passing through the MAF sensor, or anything that allows unburned air to pass the O2 sensor should be looked at.

If the numbers are high negative, the computer sees the exhaust is not swinging into the lean state often or long enough and it's trying to subtract fuel. That would be more in line with your comment about loading up. When you move the accelerator, you should see an instantaneous change in the MAF reading. Also look at the MAP sensor's reading. If it stays at a steady, constant voltage, it is being used strictly for barometric pressure. If the voltage drops to around 1 - 2 volts at idle, it is connected to the intake manifold through a vacuum hose. If the MAF has failed, the MAP could be supplying engine load data to the computer. There should be a Check Engine light on in that case.

Look at the two temperature sensors. If the coolant temperature sensor is reading too low, eventually a fault code related to that will be set. That can also cause the system to stay in open loop and disregard the oxygen sensor. I don't think that's what you have, but just double-check that it's reading is at least around 190 degrees. I WOULD look closely at the ambient air temperature sensor's reading. It is a real simple, reliable sensor. In fact, I only know of one that didn't read right, ... Because I caused it! I "bugged" it on a '98 GMC truck for my students' learning activities. Instead of the sensor, I just soldered in a pair of resistors, (I can do that because I'm a genius you know!), That made it always read 50 degrees. It started hard in the winter, and flooded easily in the summer, but once it was running and warmed up, it had a stumble under part-throttle acceleration.

The last thing to try is recording sensor data with the "record / playback" function. If you aren't familiar with that, the sensor data is held in the scanner's memory for a few seconds as it passes through. You press the "record" button when the hesitation occurs. The recording will actually start a couple of seconds before you pressed the button and continue for a few seconds after. Later you can review the numbers frame by frame to see what the Engine Computer is responding to. I apologize if I'm repeating stuff you already know.

Caradiodoc
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Sunday, April 4th, 2010 AT 8:52 PM
Tiny
TOM NIGHTINGALE
  • MEMBER
Thanks for your response.
I'm just a back yard guy that can fix about anything once it gets figured out what the problem is.

I'm about ready to drop it off at the shop and let the pro's fix it.

I'm a carpenter by trade, a real one. Not your I help a guy build a house 25 years ago carpenter.

Like you I'm a pro. I feel like the home owner fixing his own house.

Question could the throttle position sensor be bad
or the idle speed sensor

No codes again I just checked. I have a Mac small code reader.

Mech buddy has the big $6000 scanner. Friday we went through everything and couldn't find anything major.
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Sunday, April 4th, 2010 AT 9:06 PM
Tiny
CARADIODOC
  • EXPERT
That's funny. I'm a carpenter by hobby, in my 12th year of building a huge 50 x 90 workshop with two basements and a tunnel. I had three careers; tv / vcr repairman for 37 years, mechanic for 16 of those years, (electrical and suspension and alignment specialist), and automotive teacher for 9 years.

I help a buddy who has two careers. He owns a body shop, and he builds houses. I'm way better than the average do-it-yourselfer, but what you can build in a day, no, ..., In an hour, I have to think about for at least a week. I'd go broke if I had to do it for a living.

As for your throttle position sensor, the same story applies as with the MAP sensor. As long as it reads between approximately.5 and 4.5 volts, no fault code will be set. However, the Engine Computer does more than just take each sensor's readings individually. It reconciles them with each other under various conditions. For example, the TPS could legitimately read over 4 volts indicating wide-open-throttle, and the MAF sensor could legitimately read very low air flow, by weight, entering the engine, indicating idle, but they better not happen at the same time. The TPS is one of the smaller players in the fuel metering calculation but it can cause a stumble. Usually the MAF is so quick to respond to changing air flow conditions, a TPS problem might go unnoticed. Nevertheless, disconnect him and see how the engine responds.

Here's where a clinker comes in. When an intermittent contact occurs in the TPS, the voltage on the signal wire will get "pulled up" to 5 volts by the Engine Computer. (To prove that to yourself, unplug it and look at the reading on the scanner or measure the voltage with a digital voltmeter. You'll find a 5.0 supply wire, a 0.2 volt ground wire, and 5.0 volts on the signal wire). That 5.0 volts is what sets the fault code and turns on the Check Engine light. GM used to have a lot of trouble with the light coming on for every little glitch, so now the computers are programmed to sit and watch the problem first. When the problem occurs again or for a certain number of times within a specified time period, it will finally set the code and turn on the light. Some glitches might not last long enough to see on a scanner. The vehicle's Engine Computer responds faster at analyzing glitches than the scanners do. That's where the snapshot, or recording mode, is helpful. To make matters worse, there are some GM systems that display a different reading on the scanner than what you will measure at the sensor. When a problem with a sensor is detected, the computer knows it can't count on its reading, so it disregards it and injects an approximate value, from memory, based on the readings from other sensors. In the TPS story, when it is unplugged, the signal voltage will default to 5 volts, but the computer knows the engine is at idle, so it may inject a 0.5 volt signal and run off of that. The 0.5 is what would be displayed on the scanner. You would see the related TPS fault code and wonder why it is setting that one when the signal voltage is normal, Not all GM vehicles display readings on the scanner that way, but just be aware that could happen if your numbers don't make sense. I was told once that Chryslers do that too, but I've never seen it myself. If a number doesn't seem right, double-check it with a voltmeter right at the sensor's connector.

Caradiodoc
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Sunday, April 4th, 2010 AT 9:59 PM

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