TBI idle surge ?--I'm almost there, help me get over the hump!

[Goldhawg]

As a review, I had been chasing issues of low vacuum (11 in max w/low lift cam) & low compression on my 351W ever since last spring when I changed the cam and added Howell's TBI EFI. Thought I had a head gasket blown since 1&2 cylinder had low compression (~100psi) while #1 spark plug was white. No water in oil, but thought it was going out the back.

After pulling the head, the head gasket wasn't blown. Took the head to machine shop, he says head is junk with just a look--valves are sunk in. Can't be fixed. Got a set of E7TE heads off c/l, and machine shop got them right. Replaced both heads. Finally fired it up tonight. Engine feels much stronger, but the idle seems to surge. Runs great @ 1200 rpm, then will drop down to 700 and almost die. At those rpms, I have timing @ about 15 deg BTDC @ 1200, then when it falls to 700 or below, timing goes to around 8 deg BTDC. I have nice vacuum while it is@ 1200, around 19 in. When it goes way down the vacuum goes as low as 12-13 in. When it is up it will be up for several seconds before the idle falls off and then it stays low. It is not a constant quick up and down. When I step down on the gas it feels very strong and there is no missing or anything just smooth power. I know the computer needs to learn, so I ran it about 5 times in the garage only for about 10-15 min each time, fully getting up to operating temp. I don't want to drive it cuz they use salt here in the winter and I don't want it getting any salt. Need to wait for a totally dry road.

My scan tool is not getting any codes, although I'm still running too rich @ idle, with BLMs @ 108. When I step on the gas it will go up to the appropriate 125-128 but will settle back down @ 108 @ idle.

Any ideas or suggestions re the surging idle? Prior to replacing the heads I didn't have a surge @ idle, although it had other problems.

 

[rjlougee]

You didn't say which Howell setup you have, the one that controls spark or the mechanical advance type? That will help diagnosis.
Joe

 

[Goldhawg]

My system uses the MSD and has no interface w/howell's GM TBI.

Any chance it could just be bad gas? What I'm using has been in the tank for several months (at least 4, maybe 6). Very little in the tank right now anyway, so I'm going to add 5 gallons today.

 

[broncnaz]

Well it could be a number of things vacuum leak, O2 sensor to far away and not staying hot enough, idle speed set wrong. IAC might be bad, igintion timing wrong. Also it could still be a matter of the camshaft being a little to big for EFI. Many of the close lobe seperation cams do tend to have issues with idle surges with EFI but you should be able to tweak the setup. Very common to have these issues and not pull any codes. but it could just be bad gas todays gas does tend to go bad fairly quick.

Once warmed up it should only idle at around 750 rpm or so.

 

[Goldhawg]

Hmmm...I did increase the idle speed setting several months ago. That might be it; now it doesn't need it w/the good heads. I'll try backing that off a bit cuz when it does go up it goes to about 1200 rpm which is too fast. Hopefully that's it. I did buy 5 gallons of gas to add to the tank too.

I'll let you know how it goes.

 

[Goldhawg]

Changing the idle screw setting didn't really change anything; yes it would speed up a bit, and going the other way made the lower rpm drop lower. But still back and forth...couple of seconds up @ 1200 then back down to 5-700 rpm. Added 5 gallons premium and no change also.

My camshaft is the summit grind of the performer cam 472 lift; shouldn't be a problem. When up @ 1200, I have 20in vacuum; don't believe I have a vacuum leak. I thoroughly checked this prior to removal of manifold/tbi and I didn't take those apart while changing heads. Ign timing was set @ 12 BTDC, but I have adjusted it both ways a bit and still have the up/down, although it gets slightly better w/more timing (at least the engine likes to speed up.

Given I have a scan tool, how can I tell whether the IAC or the TPS is working properly? Anything else I should be looking at?

also, now that I've played w/the idle screw, is there any way to tell the "stock" setting and get it back to original? Since it didn't seem to help I'd like to rebaseline it if there is a known way to do that.

 

[broncnaz]

I think there is a procedure for resetting the idle speed with the howell setup its more involved than just turning the idle screw I think. I'm not sure on test procedures for the IAC or TPS might want to search the web for that info if howell's instruction dont tell you anything.
Vacuum leaks can be hard to pinpoint especailly if they are due to intake manifold gaskets as they can actually leak inside the engine and you cant tell. other than you cant get it to idle lower. but With EFI this is harder to actually diagnose as there are all the other factors that need to be ruled out first.

 

[Goldhawg]

I think there is a procedure for resetting the idle speed with the howell setup its more involved than just turning the idle screw I think. I'm not sure on test procedures for the IAC or TPS might want to search the web for that info if howell's instruction dont tell you anything.
Vacuum leaks can be hard to pinpoint especailly if they are due to intake manifold gaskets as they can actually leak inside the engine and you cant tell. other than you cant get it to idle lower. but With EFI this is harder to actually diagnose as there are all the other factors that need to be ruled out first.

THere is a real nice link here that helps w/engine diagnosis w/vacuum gauge:
http://www.secondchancegarage.com/public/186.cfm

Unfortunately none of mine are exactly like the scenarios at the bottom. In any case, my situation is not like a intake manifold leak (scenario 10 or 12). I think I have some sort of sensor related problem.

 

[broncnaz]

You biggest issue with vacuum readings is that your current idle is to high to be a accurate indicator of anything.