Long and Short Term Fuel Trim Question - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 10-09-2006, 01:08 AM   #1
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Default Long and Short Term Fuel Trim Question

I am new to the forum, but have had a 95 Bonneville for several years. It has been running poorly the last while and will just quit while I am driving. If I sit for a few minutes it will start back up and go again. However, it doesn't run very well'

I got me an Actrom CP9145 scanner and have been trying to determine the problem. I did not have any related codes that came in. I had one for the air conditioning (P0561)and one for the torque converter clutch (P0740) that didn't seem to be related to this problem.

I have been looking at the operating data and thought that I would start with the fuel trim. I expected to get numbers in % like 5% or maybe 25% based on what I had read on fuel trim. I get numbers like 128 for the LT Fuel Trim and 127 for the ST Fuel Trim. I also get another output called LT Feul Trim CL that was 5. These numbers were all at just above idle speed. I had to keep the RPM'* up to keep it running.

I was wondering if anyone out there had an idea on what was expected for these numbers. I read somewhere that back in the 80'* a number of 128 was the middle number for fuel trim.

Any help would be appreciated. Thanks
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Old 10-10-2006, 01:43 PM   #2
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I was able to get an answer for this question. In the 1995 model year for the bonneville (I am not sure about other years), the LT and ST Fuel Bias has a zero point at 128. Below this level is decreasing fuel and above this is increasing fuel.
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Old 10-10-2006, 01:58 PM   #3
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good to know..

Thanks for sharing that. In the newer styles.. there is 5% etc like you had originally expected to see.
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Old 10-10-2006, 06:17 PM   #4
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You mentioned it was not running well, what does the MAF, TPS, O2...etc show on the Scantool at idle or WOT.
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Old 10-11-2006, 09:22 AM   #5
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You say it will just quit while you are driving. Is that at speed or while idling at a stoplight, etc?

Any other symptoms?
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Old 10-12-2006, 01:01 AM   #6
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I only have readings at idle speed. O2 Sensor 200-800 mv, Throttle % 1, TPS Sensor (V) .53, Spark Adv. (deg.) 10, Evap Duty % 0, Evap Solenoid Off, Coolant 163F, EGR Req % 0, EGR Valv Pos (v) .78, EGR Valv Pos % 0, KS Counter 10, IAC Position 103, Inj PW (MS) 3.8, MAF (Gr/Sec) 9, IAT 102F.

The car dies at random times. If I have it idling it will die as if the key is turned off. I can be driving down the road with the throttle constant and it will die in the same manner. Sometimes I can put it in neutral while I am still coasting and it starts right back up. Other times it will take a few minutes before it will restart. It also seems to be worse when it is warmed up.

I will try to get some WOT readings tomorrow. Any thoughts would be appreciated.
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Old 10-12-2006, 05:35 AM   #7
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Quote:
Originally Posted by mnuttall
I only have readings at idle speed. O2 Sensor 200-800 mv, Throttle % 1, TPS Sensor (V) .53, Spark Adv. (deg.) 10, Evap Duty % 0, Evap Solenoid Off, Coolant 163F, EGR Req % 0, EGR Valv Pos (v) .78, EGR Valv Pos % 0, KS Counter 10, IAC Position 103, Inj PW (MS) 3.8, MAF (Gr/Sec) 9, IAT 102F.

The car dies at random times. If I have it idling it will die as if the key is turned off. I can be driving down the road with the throttle constant and it will die in the same manner. Sometimes I can put it in neutral while I am still coasting and it starts right back up. Other times it will take a few minutes before it will restart. It also seems to be worse when it is warmed up.

I will try to get some WOT readings tomorrow. Any thoughts would be appreciated.

Well those readings look normal. The important information might be obtainable when it does not start. It could either be a loss of electrical power/signal somewhere, or drop in fuel pressure. Does it crank the starter over when it dies AND cant start at the same time?

If not, I'd look into the battery cables and other electrical connections in that subsystem.

I'd also look at something with the ignition system, a common failure is the Crank Sensor or other electronics in that chain of things.

You can check for fuel pressure on the rail on these cars easy if you have a rag....just tap the schrader valve on the fuel rail. That would help you narrow it down.


I'd reseat the connection to the ICM, under the coils, reseat and clean the connections.

There is a high likelyhood this is ignition related.

So when it happens again there are a few things we can do:

Does it crank over?
Does it have fuel pressure?
Does it have spark?
Are there any codes set?

Having a scantool should make it easier to figure it out. We should be able to narrow it down with a bit more info.
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Old 10-12-2006, 07:52 AM   #8
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Sounds like an intermittent that will show all the things Hans mentions (especially when it starts back up quickly).

We've seen a lot of loose battery cables or dirty connections as Hans also suggests causing this type of thing. I'd take his advice and clean/tighten both ends of each cable to ensure good contact.
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Old 10-15-2006, 12:14 AM   #9
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Well, I have some more information now.

First, I checked the fuel pressure. It stays about 142 whether it is at idle or revved up. When the car dies, there is no dip in fuel pressure. It appears that other than a noisey fuel pump, it is working fine.

I have cleaned the connections to the battery on both ends. I have also reseated the connection on the ICM and cleaned the connection below the coils. I also cleaned the connections to the O2 sensor, IAC, TPS, MAF and the alternator. This didn't seem to make any difference.

I have tried to unplug the MAF, TPS and IAC connectors to see if this would prevent it from dying. It still died.

I have also hooked up a timing light and watch it when I am trying to start it and it is just turning over. I am getting a strobe so it looks like it is firing.

I have noticed a couple of changes today. First, the LT Fuel Trim is at 148 and doesn't change now. The ST Fuel Trim is at 128 and has quit changing. The O2 sensor seems lower now with the readings staying below 200 mv.

There are also two other things that bother me. First. When I try to run it at WOT, it will surge and not seem to get to full throttle. I don't see any fluctuation in the TPS voltage. Also tonight it started surging while at idle. This seemed to start happening about the time the LT and ST Fuel Trim went high.

Hopefully this will provide enough info for you guru'* to give me some ideas.

Thanks for your previous thoughts and help.
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Old 10-15-2006, 02:26 AM   #10
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Default Fuel Pump !!!!!!

Quote:
Originally Posted by mnuttall
I only have readings at idle speed. O2 Sensor 200-800 mv, Throttle % 1, TPS Sensor (V) .53, Spark Adv. (deg.) 10, Evap Duty % 0, Evap Solenoid Off, Coolant 163F, EGR Req % 0, EGR Valv Pos (v) .78, EGR Valv Pos % 0, KS Counter 10, IAC Position 103, Inj PW (MS) 3.8, MAF (Gr/Sec) 9, IAT 102F.

The car dies at random times. If I have it idling it will die as if the key is turned off. I can be driving down the road with the throttle constant and it will die in the same manner. Sometimes I can put it in neutral while I am still coasting and it starts right back up. Other times it will take a few minutes before it will restart. It also seems to be worse when it is warmed up.

I will try to get some WOT readings tomorrow. Any thoughts would be appreciated.
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