I have tried to tune out a drivabilaty issue the hole summer.
Im close, but not satisfied.

Combo is ( not include my bottom end )

Lt1 intake
Hotacam
headers
TF alu CNC ported heads.

The issue is low rpm driving, and stop and go traffic driving.

Under 1800rpm the car bucks and "jumps", just like it missfire, its not that much but its there and it drives me crazy.
Also throttel response is bad below 2000rpm.

I tried alot of diffrent timing tabels, aswell as experiment with fuel alot. Played with AE aswell.

What bugs me is that the WB shows a whery spiky A/F under 1800 rpm. It swings from 14.0 to 15.6, but as i build RPM the AF gets more stabel. No matter what timing or fuel I give it, the trend is the same.
More timing helped alot, but not perfect.

I played with DFCO, no change.
The WB shows no lean spike during tip in, so AE should be ok.

Right now I think the troubel is the lt1 intake and fuel distrubution troubel. I can se that my front plugs runs leaner then my back ones. Alot of search on the web also verify this probleme with short runner intakes. I also heard that the lt1 ECM has a tabel to compensate for this?

Im so close to get me a superram..


car is a 6spd 91 corvette..

oh and its better in open loop than closed loop. Not perfekt but better...

 

Before all else fails, try plugs and wires. A slight problem there can cause low RPM/higher load problems that won't show up elsewhere. Cracked porcelain, barely arcing wires, etc, will arc more when the piston is closer to TDC due to high cyl pressure, such as low RPM high load. Not saying it's your problem, but could be.

 

This is totally left field but are there any sort of proportional control routines in your ecm that are meant for use during cruise with a manual? Might be worth looking into to see if there was such a routine implemented. Even the lowly tbi cars had some form of control for the idle. The proportional control could be a good way to control that, or the gain may be off.

Another possibility is the PID stuff for the O2 is off. I had this with the ol' tbi ecm and it would be a rolling surge/bucking.

How does the VE table look? Are there any sudden changes in that area of VE map wise that can cause the AFRs to ocillate? Sudden changes in AFR cause power impulses when your leaner then stoich since the engines power output changes so rapidly there.

IMO I doubt the SR would help much. This sounds like a global fueling issue during low RPM cruise.

 

Thanks for the tips..

I will post my VE tabels and spark tabels later today.

I will try new plugs, new wires.

Could you please explain more about "PID stuff for the O2 is off" ? Dont follow you there.

And there is no speciall "proportional control routines" what I know off, perhaps someone else know about this??
The never had this probleme with the old TPI setup and the hotcam, it whas after my head/intake/headers install this started.

I also kan se that my front plugs run leaner then my back ones. Even tried to swap injectors, no change.

 

Chapter 5 in the tuning sticky above.

Under the 730 stuff look at 02 Constants and Stoichiometric postings as far as the 02 stuff is concerned.


Taming the IAC steps at low rpm and associateed throttle followers has a great effect, especially at ow rpm on peaky cammed engines at low rpm ranges. Look under the sticky for a post I started entitled Throttle Follwer $8D.

later
Jeremy

 

So as littel IAC steps as possible?

I woulde like to explain the behaviour my car has a little further.

Its in low gears , first,second and a littel in third, and below 1800rpm. The car buks,feels just like missfire, and is very sensitive on the throttel.
Abow 2000 the car is supersmooth.
Its better when the car is cold and in open loop.
The WB shows that my AF is not smooth at those rpms, and its nothing i can do about that because the car goes in closed loop, so add more fuel just change the BLMs but the AF is the same..

Its better if i go uphill at those rpms and put som load on the engine. I think thats because I open up the TB blades more, but i dunno.

I use 28 degrees timing at idle, that helped alot. I also run around 35 degrees cursing timing at those rpms. I tryed both lower and higher but 35 worked best so far.

 

Try run open loop or shut off BLM to see if It want to go richer or leaner........

A good Idea can be to buy a romulator

/N.

 

yes that could be a nice test

But what bugs me is that this is far from a wild engine combo. And should be "easy" to tune. I never heard of this kind of troubel before with a hotcamed short runner intake 350.

 

I dont know the part specifics enough to know based on what youve listed but do you have an EGR?

But had a stock GM pickup that did that type of bucking/surging at odd times. Took me a year to track it down, ended up being pin hole leak in EGR.

 

Does LT1 have EGR? I don't think so, but yeah, EGR problems cause VEERRRYY similar problems. Did you adapt an EGR on there?

 

The EGR "holes" in the intake where pluged. And I disabeld the EGR in the chip..

 

I know that my DIST is a bit loose. I hade to chims it to get all the play out. My pickup coil is new, same as the ign coil and ign module.

 

send me you bin and log files. I would like to look at everything.

Thanks..

 

Thanks I will..

Thanks for taking your time everyone..

 

Be careful with LT1 intake installs. I went through this last year. I use stock GM 882 heads. The LT1 intake DOES NOT cover the EGR holes completely and allows exhaust to leak into the engine with GM smog heads.
Some people claim that the intake gaskets block the holes but it is not true. I bolted an LT1 intake to a cyl. head, flipped it upside down, and you could see a 1/4" gap AND the intake gasket has nothing backing it on the intake side so the exhaust pressure can blow it right out. I had to alum. weld angle stock in to make a backing for the intake gasket.

Not sure about the TFS heads EGR port. I am not saying that this is your problem. Just something to check.

The scary thing is that a lot of people are running LT1 intakes this way with stock GM heads. I wonder how long the engines will last.

J

 

Yes, the LT1 intake does have EGR on the back of the manifold.

 

I use old style TF heads, so thay dont have EGR ports in them.

 

here are my bin and a scan file (datamaster).

focus on the low speed low rpm areas, where i have my trouble.
at the end of the scan i cruise slow, and got som off-idle crusing aswell.

If you need I can make som more scans when the car just does slow speed/rpm crusing.

thanks


http://www.vetteforum.org/attachment...4&d=1124122856

 

there is alot of part throttle timing there. I would like to rework your exising spark table. IMO, it'll help unless you have a mechanical issue. Should be able to mess with it tonight. That's if you are interested..

..otherwise reduce part throttle timing from 41*.

 

I would appreciate it alot!
Thanks.

 

devilfish,

What head gasket did you use?

 

ohh have to ask my engine builder about that..
what could have happend?

 

Just went thru this. It seems that FelPro 1010 gaskets are prone to a defect. I have pics posted in this thread.
http://forums.corvetteforum.com/show...64&forum_id=48
I had some similar tuning issues for this past year. Trying to track down an oil leak, I pulled heads and found that gaskets were failing.

 

You say in that topic that you got" stumble at idle although cruise seemed to be fine", my idle is fine. Is the low rpm cruise thats not smooth But thanks for the heads-up, Im going to need all the tips I can get.

 

Have you been taking notes, and looking for trends?. If you *just* tuning without studying what you're doing you (as well as anyone else), can zoom right past optimum without knowing you were there.

Yes.


From what you've said, I'd be looking for a mechanical problem. When you change everything in the code, and nothing works, that to me would indicate the code ain't the problem (or you just missed the tune [see my earlier comment about notes]). Or that you might need to make a code change, but in that case why hasn't anyone else had your problem?.

Maybe you can ask around about how well a LT1 runs in batch fire mode to see if the cyl correction really matters.

 

The part throttle S/A really stuck out like a sore thumb. It's not the greatest idea to run 41* 90% of the time. So I reworked the spark advance table and sent it to you. Hopefully that helps. If not then I agree with Grumpy, that it could be an electrical/mechanical problem.

 

Although your symptoms may not match mine, I would at least determine that these particular gaskets hadn't been used. That was my point. Listen to what Bruce has said. If the changes in the tune don't make any difference then there must be something else happening. That was the case in my situation.

 

a leakdown test could be done. That's better than not knowing untill the heads are off......

 

11so,

In the case of these gaskets, leak down tests may not show there's a problem at that point in time. Mine were on for 5 years but they eventually failed. Based on the feedback I got on the Corvette Forum, I'd say if you have the FelPro 1010 on there now, change them because they will fail and it will be gradual.

 

Have you tried open loop with richer/leaner AFR yet?

/N.

 

did you try the bin I sent???

 

I have not tried richer/leaner AFR yet. I have a ide im working on.. I keep you posted.

11sORbust, I going to try that tomorrow. Thanks for the bin..

 

Probleme solved! You cant belive how good I feel right now.

Here is the solution.

There where several things I changed that made my car run great.

The main fault whas the PCV system. I routed the PCV systems as I whas told for my lt1 intake. ( SEE PIC BELOW ) This dident work for me. I dont know why, perhaps it has something to do mith my heads that dont have EGR ports in them, but i dunno?
After I pluged both of those holes, and routet my passenger side pcv system to free air it whas like driving a hole new car. I cant belive how much smoother the drive is, and I also got rid of the anoying popps in the exhaust after i let of from a WOT run, GREAT!

This made the car go 90% as I whanted.
Next I borrowd a romulator from a friend ( damn this made tuning alot mor fun and more easy ).

I read alot about the IAC on this site. And I lower my IAC slope gain to 75%, increased the MAX IAC steps to 231, this made my off-idle throttel to be less sencitive, no more "jumping" starts from red lights.
I also changed the TPS setting for enabel DFC ( a tips from a old thread from Grumpy )
All this made the car run 99% good under the "magic" 2000rpm

I also tried the bin that 11sORbust sent me. The bin did not do to much to "fix" the abow probleme ( this is pretty obvious because it whas a 95% mechanical probleme ). BUT it did solve som other issues I hade, higher rpm knock counts, and som tip-in issues.
I just had to change your timing tabel at some areas to achive great results, good work 11sORbust!!


Thanks everyone that gave me all the good tips, this site ROCKS!!
If it whasent for you I probaly hade give up and got me another intake, or wasted my cash on something esle.

regards

Devil...

 

Yes, I can.

Glad your happy.

 

Now I just have to decide if I should investigate why this happend. Or if I just should get out and drive the rest of the summer

 

Glad you figured it out!

Drive, drive, drive!




Keep in touch!

 

hehe yes drive drive drive!

 

That great, was my pleasure to help. You shoud also see lower coolant/oil temps now too....




...could you send me your stock bin?

-Tim