Idle timing on a cammed 97 L31

**silvergs** · 08-02-2015

Got a customer that dropped off a 1997 1/2 ton that he rebuilt the motor on and installed a "Thumper" cam. Bad choice on his part but I am trying to get it to idle and run halfway descent. My issue is at an idle it suddenly starts pulling timing and it gets down to about 0 advance. My target is around 28. What conditions will pull that much timing at an idle? He says that the oil pressure guage is wrong because when it warms up it drops to 5 psi and the check guages light comes on. Will this do it?

Thanks
Mike

**fastacton** · 08-02-2015

I don't believe the PCM has an oil pressure input, so that's probably not it. I'm not very familiar with the black box PCMs, but I'd check the ignition module to ensure it's working properly and look for knock retard as well as just seeing what the timing table is set for right now.

**Fast355** · 08-02-2015

Originally Posted by silvergs

Got a customer that dropped off a 1997 1/2 ton that he rebuilt the motor on and installed a "Thumper" cam. Bad choice on his part but I am trying to get it to idle and run halfway descent. My issue is at an idle it suddenly starts pulling timing and it gets down to about 0 advance. My target is around 28. What conditions will pull that much timing at an idle? He says that the oil pressure guage is wrong because when it warms up it drops to 5 psi and the check guages light comes on. Will this do it?

Thanks
Mike

My buddies 383 had issues with the Crank/Cam correlation and it would put his truck into a default timing mode. It would run at about 0* around idle and jump up at about 3,000 rpm to about 22*. I would start by checking the CMR with a scan tool. Get it set correctly. It has been so long since I have had a black box on my 97, but may have to perform a crank relearn as well. I know on my brothers 99 Suburban it made a noticeable difference once the relearn was performed on the fresh engine. Might also be some kind of MAP related test throwing the PCM into limp mode. I would put a mechanical gauge on the engine to test the oil pressure, no sense in trying to make an engine run well that is dying anyway then getting blamed for it.

**Fast355** · 08-02-2015

Honestly not sure how that Thumper is going to work out with the Blackbox. He should either consider an 0411 swap or drop back to a cam with less overlap like the LT4 Hotcam. LT4 Hotcam will run very well in these engines if you degree it in on a 107 or 105* ICL rather than the 109* ICL it drops in at. At 218/228 @ .050 on a 112* LSA it is fairly PCM friendly even to the Blackbox.

**Fast355** · 08-02-2015

One other thing, honestly speaking as restrictive as the stock intake on the L31 is and as limited as they are in RPM, a production LT1 or LT4 Corvette cam works about as well as anything else. I put a production LT4 cam in my Express van with the Crane Gold 1.6 full roller rockers and it would pull well all the way to 5,200-5,500 rpm where the intake gave up anyway.

**silvergs** · 08-03-2015

Thanks for the feedback guys. I know its a bad choice for a cam but it is what he put in.

I have the idle timing set at about 30 but it never gets over 10 or 11. Don't know why. I will hook the scanner up this week and see where the CMR is. I'm just trying to get this to run a little better so he can go and finish up his end of it.

Mike

**Skinny Pedal** · 08-03-2015

Can you post up the tune?

**1project2many** · 08-03-2015

For reference, the HT383 cam is similar in lift and duration to the old 350 hp 327 cam, which is a mild cam that boosts low and midrange power well. They can be had for short $$.

The thumpr might be fun to tune. Probably stock converter and a/c equipped, too. That would definitely be fun! With a loose converter and well tuned dynamic idle settings it might not idle too badly. You might be dealing with surging when idling in gear. The stock settings bring timing up fairly quickly if the idle speed is a little low. Make that window bigger and you'll be happy. Open loop idle would be nice... toss the corrections out the window when they're hardest to nail down. MAF sensor giving wrong readings due to intake pulse reversion? I wouldn't expect it unless the cam really lopes but I'd watch for it. How about SD fueling when MAF is down? Back to trying to tune with MAP bouncing around. I don't even believe black box definitions give access to MAP filter parameters? How about an 0411 swap?

**Montecarlodrag** · 08-03-2015

Is it throwing codes? There must be a correlation between the cam sensor and the distributor advance. If there is too much variation it will not work right, it may end not even starting.
You may need to set the correct distributor timing until you get +/- 2 degrees of cam offset. Timing can't be adjusted with the distributor, you can only set it to the correct alignment.

Also make sure the PCM has the correct Knock module, it will not work right without it.

**silvergs** · 08-04-2015

Here's the latest file. I just can't figure out why the timing is dropping off so far at an idle. Haven't gone through much more than that at this time.

**Skinny Pedal** · 08-04-2015

Thanks.

What rpm are you seeing at idle and what KPA?

I don't see where this cam would even be close to 28* at idle with what timing you are commanding. KPA should be pretty high at idle with the thumpr cams.

Whats the IAT and ECT also?

I can see where this cam would be idling about 12-18 degrees minus 2-7 depending on corrections being made.

There is also 10 degree for RPM error. WIth the thumper there is always RPM error. So now with the data shown minus for correction I could see where you could be near zero.

I'm not saying its not a correlation thing either. Thats very important on the L31s.

EDIT - The minimum allowable advance vs rpm vs map - for where this engine will idle with that kpa allows 8 degrees retarded.....

**silvergs** · 08-11-2015

Skinny Pedel hit it. I didn't have the timing up far enough on the high KPA side. It started to come around and then suddenly got a no start issue. Well after a lot of messing around with intermittent spark I remembered a pending code that had shown up earlier. Crank sensor. Did a little checking and then called the customer. He had installed a double roller timing chain in this thing so the trigger only covers half of the sensor. Wiggled and "cocked" the sensor slightly forward and started right up. He has now picked it up to replace the double with a single and will drop it off later for me to finish.

Thanks for the input guys!

**Skinny Pedal** · 08-11-2015

Sweet!!

I'd still like to see where and how its idling. Please share the tunes as you go. That engine can take some more timing too......

**Fast355** · 08-11-2015

Originally Posted by Skinny Pedal

Sweet!!

I'd still like to see where and how its idling. Please share the tunes as you go. That engine can take some more timing too......

^^^The L31s like alot of timing, especially with larger cams. I could not open the tune posted here because I use HPT. Stock LT1 F--car cam in my brothers Suburban really liked the timing when I threw this map at it.

My 215/220 @ .050 on 114° LSA can with Aluminum Etec 170 heads LOVES timing advance. I bet the timing could cone in even quicker with a Thumpr cams tight LSA.