454 Timing

krvoorhies

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I have a '98 GMC 2500 Suburban - have extreme drivability issues no more than 20 mph up minor incliines. Has a 454, was doing fine but just in one day it went from full power to almost nothing.

I tried a new fuel regulator and a new fuel pump, no change (but did improve the fuel pressure a little). I know that you use the scanner to set the timing (cam offset), but I noticed the dampner has a timing mark and the the timing gear cover has a tab with a single pointer. I went a head a ran the old timing gun to see, it was firing about 25 to 30 deg BTDC, but went reved to 2000 RPM's, no change in timing location (no advance).

Smells rich when idling, but idle is no too rough. No unusal mechanical noises, but does sound like backfire when underload.

Anyone seen this before?

Thanks for any help.
 

Jim Fairbanks

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Did you change the fuel filter too?...Is the check engine light on?...Are there any codes stored?...A sudden change in the way it was running might be bad plugs or wires...Or even bad gas...You could have gotten some water in the gas....Jim
 

kev2

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the Crank position sensor and the ECM/PCM set ign timing the CMP cam sensor is referance for injector timng..do not move it, it willnot adjust timing.

to add to colleagues ideas, any modifications? fuel would be first guess for a sudden change if you can relate a refueling and problem.... also exhaust back pressure.
 

krvoorhies

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Thanks for the suggestions - I did fill up about 30 miles prior to problems. When I had the tank out to change the fuel pump the fuel looked clean & no funny smells. I even put about 10 gallons in my other car - seem to be running fine.

The thing that gets me is that the idle is fine and upto about 1500 rpm - then it starts missing/cutting out. No check engine light, I had one a couple days ago, but the light went out.
 

Jim Fairbanks

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If the check engine light was on then there is a code stored....Go to Autozone and get them to pull the codes and post them back here....Jim
 

krvoorhies

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Jim Fairbanks said:
If the check engine light was on then there is a code stored....Go to Autozone and get them to pull the codes and post them back here....Jim
I scanned and got the following code P1351 "Ignition Control Circuit Voltage High". There are two occurances one is Pending, the other is "with Freeze Frame".

I replaced the Ignition Control Model about 3 weeks ago.
 

Jim Fairbanks

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Check the wires going to the vcm ,make sure the connections are clean and tight and the wires going to it are not broken or chafed,then reset the code and see if it comes back...Jim........
 

krvoorhies

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Jim Fairbanks said:
Check the wires going to the vcm ,make sure the connections are clean and tight and the wires going to it are not broken or chafed,then reset the code and see if it comes back...Jim........
I checked the connections, they seemed fine. I looked up the P code in AllDataDIY and followed the troubleshooting chart - one funny thing is it said to check the output from the VCM to the ICM in AC-volts. When I did that it exceeded the allowable 6.5v out of 1-4v. But with it set to DC it was about 3.3v. I also noticed that another troubleshooting guide for the Enhanced Ignition Testing shows the same test but in DC.

Which one, AC or DC, makes sense. I am hoping DC, because the AC doubles the voltage and causes a VCM failure according to the chart.

If I go the DC route, then the chart drives me to a ICM failure (no surprise - even though I just replaced it about a month ago, its from the good ole corner store). I am going to try replacing it and I will post back if anything is better.

Thanks for the help. I cant figure out how to attach files in here, but I can .pdf the charts and send them if that helps.
 

brcidd

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My '95 454 dually acted similiar- I found that the oil pressure sending unit was arcing (shorting) internally (after finally seeing oil pressure gage flucuations) The oil pressure sending unit includes a switch to the fuel pump- which was not receiving proper voltage due to the arcing and shorting of the circuit- the test was to unplug the oil pressure sensor- to see how it ran then- the bucking and backfiring quit immediately- that circuit is a parallel circuit- it will run off the fuel pump relay anyway-- those 454s generate a lot of heat and heat kills that pressure sensor- I have seen two ther 454s have the same issye...........yours may be suspect.
 

krvoorhies

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Thanks for the suggestions:
I have tried disconnecting the oil sender unit, but no change.
I replaced the Ign. Control module, still no change. Did clear the P1351code, and it has not came back.

I noticed something else tonight, when I shut off the engine I hear air bleeding down, like a nail hole in a tire, around the engine, I just can't pinpoint. Someone mentioned plugged catalytic converters, could that be related?

Not sure where to go next - it smells rich, getting spark through all the wires, good fuel pressure even at 2000 rpm. Removed air filter to check for any intake obstructions, but runs rough.

Still open for suggestions if anyone has any left. Thanks.
 

ironhead

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It could very easily be a converter. You can remove the converter to be sure, or you can remove both front oxygen sensors. This will set codes and it won`t run properly, but if it is a plugged cat, the open holes from the sensors will let it breath enough that you should see a drastic increase in power.
Guy
 

krvoorhies

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Guy,
Thanks for the input, I am going to give that a try tomorrow. I ran out and did a vacuum test I read about on the internet. I picked up a vacuum port on the manifold, at idle it was around 25, when I brought the rpm upto 2000 it went way down, almost to zero, which is supposed to mean a lot of back pressure.

When it lost power, it was gradual over about an hour of driving - nothing instantaneous, like a module/sensor failure or mechanical failure. I'll post back what I find out tomorrow. Thanks.
 

340x

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I just worked on a 94 Dually 454. Timing chain jumped forward, kind of odd. Usually it goes backwards. It jumped enough to bent all of the intake pushrods and destroy 2 lifters. It wouldn't even start. It could possible be one tooth off and still run but it won't have any power. This one had 170,000+ on it. It was way past due. Typically 100,000 miles is time. The cam gear usually is aluminum with nylon teeth. The nylon starts chunking off and falls in to the pan. When enough of it falls off it will jump time. Sometimes a little, sometimes a lot. It is a cheap fix, $50 for parts. Plan on a good day to tear it all down and put it back together.
 

krvoorhies

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340x,
I thought about the timing gears, but I would expect to get some troube codes maybe related to camshaft, crankshaft position sensors. Anyway that might be the next path. Thanks for your help.
 
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