Hey Guys, So I'm starting to get the hang of the whole tuning thing. I've got my setup running pretty good now. Problem I'm currently fighting is a stumble when I go from P or N to D or R. The engine stumbles pretty hard then recovers. Also weather or not it is related I have a surge at idle. However only sometimes and more noticeable when engine is cold. When its warmed up and I pull up to a stop light it begins to surge and I can feel the brake pedal pulsing up and down from the power boost vacuum signal. If i have lights, stereo, or heater blower on it is way worse with the increased load.

Tuning so far, I've got it running between 126 and 130BLM, only a few knocks higher up in the RPM that I'm trying to take care of. AE is close. EGR, PE, Knock Retard, DFCO all disabled in attempt to get a handle on fuel and spark.

Min. Air adjustment has been done. A note on that however, I have min idle in the chip set 650. When I set Min. Air I can get it down to 500RPM. When I shut down, reconnect the IAC and start up it is about 650. However if I go for a drive my idle won't come back down. I usually idle around 900-1000RPM. Is this normal, I have had previous issues with high idle (1600RPM) which I thought I had solved.

7477ECM
350TBI Conversion
Slightly Higher Than Stock CR, Edelbrock Performer TBI Intake, Hooker Headers.

Thanks! Any help is greatly appreciated, Oddly this not so extreme build is a real pain to tune.

 

First thing I would look for with those symtoms is a vacuum leak...

 

Thanks for the reply!

Sorry I knew I forgot to mention something. Seeing as I've been fighting the high idle for a long time I have checked for vacuum leaks multiple times. Ive visually checked, used a can of TB cleaner, as well as a vacuum gauge and I believe got a reading of about 17in/Hg. I can repeat those tests just to be sure but have never been able to find any. Also immediately after I do a min. air reset I obtain I steady idle at 600...until I touch the throttle and then it never comes back down. I've been thinking that if it was a vacuum leak it wouldn't idle at 600. Is that correct or could a 350 idle at 600 decently even with vacuum leak? When I say decently however, I wouldn't attempt to drive it down the street like that.

What do all of your guys cars idle at?

 

Not sure I can answer that... but a guy just posted this link on vacuum leaks yesterday in another forum and it may help you...

http://www.aa1car.com/library/vacleak.htm

 

Hey, yeah that other guy was me haha. I was doing some research and came across it... It pretty well has convinced me that I have a vacuum leak. It just a matter of finding it, I have spent hours on end trying to find any. I'm currently going to school for Auto. Tech Level 1 and the school is going to let me borrow the smoke machine over the weekend. If that doesn't come up with any leaks I'll be lost.

Anyone have any other ideas what could be causing these symptoms? What do your cars idle at? cam? intake/headers?

Thanks Again!

 

Do you have any data logs?

Since it's a conversion on an 81 vehicle I could only guess you have a voltage problem... if the charging system and batterey are taxed hard and your voltage is not steady then neither is fuel presure or injector pulse etc... may be something to look at.

 

I like that idea, hadn't thought about that. I do have a brand new battery and alt. However still a possibility. After I smoke it I will hook a fuel pressure gauge back up to it and go for a drive.

Do I ever have data logs.... 100+ Ill post my latest if you feel like looking at it...

Looks like voltage is good until I stay in drive but the vehicle is stationary like a drive through. In my data log voltage drops to 12 and 13 in that situation. Question is, is voltage low because of the slow surging idle causing TBI issues or is it a vacuum leak causing poor idle causing low voltage.

Good example of the surging is sample 219-249 there I have no TPS change, all in oark and idle goes as low as 500 then up to 1000 to recover...

Thanks for the suggestion I will look into this weekend!

 

Well your fueling looks spot on. Voltage is fine. Takes a long time to warm up? And then only to 186ish, what does the chip call for? Could be a 195 thermastat meaning your still in choke. Goes open loop in idle but has good solid idle around the 1818 line. Look at your TPS, it's .49 and IAC is 45 or so. I would give the min air a half turn in for that and try to get closer to .54 TPS and that will bring IAC counts down. Then back to what the chip calls for as far as thermastat.

 

Hey, So smoke machine found no leaks. Will adjust min. air tomorrow! Thanks for that suggestion. As far as heating up it does seem to take quite some time. My fan clutch is seized and I'm working on an electric fan install to come shortly. I'm wondering if the fan being stuck on could keep it 10 degrees too low. I have a 195 stat installed. You said see what the chip calls for? Where would I look for that? If you'd like to see what I have going on in my .bin I'll upload my latest. Thanks for all the help!

 

Hey, So I adjusted my min. air to a TPS reading of .53V. I've got a new idea on the problem.

It starts fine but a few minutes into idle it starts to surge. This stops once the engine is run up to operating temp. Being that the fan is stuck on and constantly sucking cold air into the engine bay could this be throttle body icing? As for the stumble when its warmed up maybe this is due to the fan keeping the engine in "choke"?

What's everyones opinion?

Thanks!

 

im having the exact same issue and have not done custom tuning, i have a factory 350 chip just reprogramed for a 185 stat, was fine for 2+ years after doing that, the surge is recent, i just replaced the IAC motor today.. did it right away. im going to adjust min idle air in throttle body when i get a chance and see if that helps, and then scope the TPS sensor and see if it has a dead spot

 

Dippo,

I have yet to solve my stumble/surge issue however I think mine may be more related to timing.

Some advice for you... Is your engine modified at all besides changing the stat? First impression from reading your post is DON'T adjust min. air. If it has worked fine for 2+ years it shouldn't be causing any problems. I would look for other issues first before adjusting it.

Have you done any recent repairs? Any intake gaskets or seals? Could be a vacuum leak either from a failed repair or a failed gasket.

Checking your TPS would be good to rule it out.

Just remember that if it worked fine for that long and now its giving you issues something changed/failed to cause it. Or if you have done any mods before it started then that would be something to look at.

Let me know what you find.

 

well 1st thing you should do is to check the TPS for dead spots and then the location of the throttle blades after you decelerate. and then what I think the problem is the IAC is bad, set incorrectly or sticking. and hop this is the right thread, LOL if it diesels when you shut down, then is sounds like air is getting past the throttle blades.
hope this helps
Happy tuning! Speed Safety

 

brake booster

 

J Makk, could you explain your idea a little bit? Your saying a leaky brake booster could cause this?

Thanks for the suggestion!

 

So this is on a 'stock' engine except for the intake? I know headers will require a bit more fuel but not so sure about the intake however not at idle. Those smoke machines seem like they make even the smallest leak seem very big. I'm assuming the converter is stock to the vehicle as well? I had a horrible time trying to get around the stock converter at idle with the cam I picked. Needed idle to make power to fight the converter but converter would just fight back harder. If I went to low I got a MAP flag and died (never looked into getting around that).

Brake booster or purge canister(if equipped) would be some places to look. Is it possible to smoke them as well? Don't forget the vent controls inside vehicle. I'm sure you've pulled vac. to just about everything but the brakes by now, but mentioning just in case you have not.

 

TZFBird, Thanks for the reply!

Since installing headers I have re-tuned the main VE table. Just occurred to me I haven't done anything with the other fuel tables. Maybe richening it up a bit would stabilize it?

The converter is stock, not sure if I had mentioned I had a very mild RV cam in it. Specs are found here...

http://www.mellingselectperformance....haft-specs.pdf

I was under them impression that only big lopey cams would need a deferent converter is that usually the way it goes?

I hooked the smoke machine up to the booster supply. Seeing as the 7747 doesn't have a purge solenoid this should have made any EVAP leaks visible shouldn't it have?

I should mention I also have a brake pedal "vacuum surge". When applying the brakes lightly and slowly coming to a stop, if I just lightly push on the pedal enough to keep the wheels locked when I stop, the pedal moves in and out as the engine surges. I'm assuming this is just due to the vacuum supply to the booster being lost when the engine goes into the stumble part of the surge. Does that sound right or could this be pointing to a bad boster?

Thanks Again, Its great what everyone can learn on this forum!

 

allright so after finaly putting in a little time to diagnose the problem i found my IAC (idle air control motor) wasnt working properly due to all the carbon in the bore where it sits. and there appears to be a bit of a learning curve to it as you saw i posted earlyer that it didnt fix the problem, but after about 2 or three starts it was working almost perfect and then clewaning the carbon made it 100%.. 56 bucks at pepboys and problem solved. your problem sounds the same as mine. id check your IAC. its on the left underside of the throttle body. they get all carboned up with crap..

 

Dippo, Glad you got it fixed, amazing how expensive the IACs are for what they are. Ive also replaced mine and cleaned mine within the last year and it is still nice and clean. Unfortunately its not the issue this time.

As soon as I get time between studying I'm going to try and richen it up in the other tables that I haven't added fuel to yet. JMakk or TZFBird, have anything to follow up with?