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TBI Trouble

DavidB

DavidB

1/2 ton status
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I know a few basics from TBI trouble over the last couple years, not enough. Posting to get some help and get all my thoughts together.

Thanks!

Working on my buddy's rock buggy.

Had a JY 350 when he bought it. Had/has rich/rough idle and stalling issues. Engine blew a HG, got a crate 350 from Summit - not a TBI engine, ordered the wrong one and got a carbed engine. Modified manifold bolt holes for TBI manifold....might have vacuum leak, didn't have any starting fluid to check for one. Anything else we should have done for the carb to TBI change? Cam?

Painless harness.

No VSS...OK?

New with new engine Holley ?670? TBI. Needs more fuel pressure?
Fuel Pressure is 11 right at the pump which I think might be low, but that's all the pump's got. Might try another. No regulator between pump and TBI, is there supposed to be one? Is there one in the TB itself???
http://www.cfm-tech.com/gm_tbi_fuel_pressure.htm
Trucks with 11-12 (or lower) psi will experience a drop in power, detonation, and/or drivability problems when any airflow inducing modification is made. Typically the product added is blamed, however, it is actually the lack of fuel that is the real problem! We can't stress how common this is!
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Shorty headers and a hushpower muffler

Wow, lots of TBs from Holley. Looks like he replaced an earlier model 502-6 style with a later model 502-9. He said he had to do something to get the choke to hook up...? Wonder if it caused other issues? Maybe the ECM doesn't like it?
http://www.holley.com/types/Direct%20Replacement.asp


New distrib.

Single wire 02...probably going 3 or 4 wire. What's the diff 'tween 3 and 4?

Coolant temp sensor has been spliced in I think a yellow and orange wire. Blue wire was cut off...? :confused: I think that has some control related to the IAC's function. IAC looks like it's plunged all the way in to the TB all the time. IAC might need reset, I need some good inst. on how to do it.

Engine temp is controlled by fans w/ switches. I'm pretty sure TBI is pretty picky about temp. Should be 210 right? Probably need to get a thermo on there for the fans.

Timing Vac/light - didn't have a timing light so we tried to set it with a vac gauge. Can't get much more than 15" vacuum, and it bouces a few inches up and down....vac leak?
http://www.secondchancegarage.com/public/186.cfm

Come to think of it, I don't think there's a fuel filter on the stupid thing. Wonder if the new TBI got jacked up by some junk in the line from when it was all apart.....

Have a scanner but still learning to use it. I don't think we have any directions for it.

No EGR/Smog stuff. Off road only.
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In short the cam and also the VSS are your most probable issues. The cam freaks the ECM out since i'm sure it's probably some mild/hot cam NOT designed for TBI engines and the VSS is usually the cause for the stalling issues. Fuel pressure should be 9-13 psi measured AFTER the filter and there is a regulator within the TBI unit itself.
 
Thanks, how's it goin'? I'll put that in my notes. Didn't get a reply notification so I thought I'd stop by and check. He ordered all this junk from Summit, you'd think if the cam wasn't going to work with the TBI they would have said so. As far as the VSS, I'm not crazy about this Painless setup without it. Heck it actually might be there somewhere but the guy that built the buggy might not have put it on. Where would we get one and where would it be installed?

How about the 3 wire coolant temp sensor converted to 2?:rolleyes: Is the third maybe for the factory temp gauge? Has a mech gauge on it now.

I want to test a bunch more things, maybe go 4wire 02. Haven't even checked for vac leaks yet. Holley TBIs seem to have a rep for leaks.

I'm gonna shoot some pics and post I think so people can have a better idea of what I'm working on in case I miss anything.

What's the deal with 4 wire vs 3 wire o2? I hit the header with my laser temp light and it's not 650* which is what the o2 requires if I'm correct. Also, do you know what applications they came on? This one is way cheaper than others I've seen, wonder if it's any good. I've read to only use Delco parts for TBI. http://www.lmperformance.com/2595/1.html
 
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Whats the ECM from? A bunch of folks on here with the 1227747 ECM have said that VSS has no bearing on their idle, while my *TPI* runs horrid without it. I don't doubt their experiences, but if you've got a later ECM, it might be more critical.

Never heard of a 3 wire temp sender. Two wires for the ECM, one for the gauge. Wouldn't surprise me if there were combo ones out there, but the wrong wires won't work the sender right for the ECM. A scanner would tell if the ECM is seeing "correct" temps.

Get a thermostat in there. TBI isn't necessarily picky about temp, but if it's all over the place (instead of varying slowly, or staying constant), it WILL affect how it runs.

O2 sensor isn't a likely problem with everything else you've mentioned IMO, so I'd hold off on that until these other issues get figured out. Cold, the O2 sensor has no bearing on idle, so if you've got poor idle when cold, something else is wrong anyways.
 
I don't know about the ECM. Supposedly 88 blazer. Scanner says VSS is "off".

Will check temp gage read with scanner.

Has a coolant thermostat but the fans are not thermostatically controlled, they're switched so temp kinda varies.

As far as the o2, it runs rich until it kicks in = goes into closed loop? Then it gets leaner but runs like crud both ways.

Idle/vacuum bounces, won't pull more than 15" vac.
 
Cam = stock cam for carbed engine application. GM crate engine around 200 hp. Shouldn't be anything hot.
 
DavidB said:
Cam = stock cam for carbed engine application. GM crate engine around 200 hp. Shouldn't be anything hot.
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With a vacuum reading that low it either has a vacuum leak or a hot(ter) cam than stock. A bone stock cam will produce close to 21" vacuum.
 
Fixed some vac leaks (hoses) and it ran better but surging. So he reset IAC. Now it won't run at less than 2k RPM. :doah::confused::mad:

Scanner says 145 steps on IAC just like it did before but the plunger is not visible and it was protruding before.
 
Reset computer, back to idling/surging. :mad: Will not run without IAC so that it can be reset....that means....?

Bump.
 
I'm hesitant to blame the IAC, but since you say the counts are the same but it's position is different, that would seem to indicate something is up there. Has it been cleaned? (procedure on thirdgen.org IIRC)

Would still seem to be a vacuum leak, TBI base gasket?
 
It's all brand new.

Pretty sure we don't have any vac leaks. If we make a leak, it runs better.
 
So maybe I've got it backwards...is there an indication it is running rich? Higher IAC count=more air? Been awhile since I've dealt with IAC.
 
Weeeeell...

looks like the guy that built the buggy never put a ground strap from engine to chassis. :rolleyes: At this point, it looks like that was the issue, but it hasn't been run much yet. Will see.

It was running lean/rich.

Thanks for the help, I may be back for more! :bow:
 
I'd be curious to know what the difference was with the ground strap on and off. I have a very small surging at idle as well and no ground strap on the frame yet.
 
It wouldn't run at all with the driveshafts disconnected. So it was getting a little intermittent ground through the drivetrain. I imagine faulty grounds would cause all kinda of problems.
 
Well, so close, yet so far. Now it idles great but stalls immmediately under load. Help very much appreciated.
 
Are you saying park/neutral idle is ok, once put in gear it stalls, or that it stalls only when the throttle is applied?
 
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