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Old 11-06-2012, 08:30 PM   #1
phucheneh
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Default Anybody done GM TBI conversions (or just know them well)?

These things have always been dead simple to work on. Personally, I don't think it's really any improvement over the feedback carb, and would rather just have a Q-jet and an HEI with vacuum/mechanical advance.

But I've got a truck ('95 1500) that is overfueling to the point of near-hilarity. If you take the air cleaner off while it's idling, you get a fog of gasoline from the injectors spraying against the closed throttle blades.

It appears that someone took a late 80's motor and used it to replace the stock engine...both TBI, but for some reason, they had to do all kinds of wiring butchery. I can't do flash codes because the MIL won't illuminate...I guess I can just hook a test light up the MIL output on the ECM, but haven't done so yet.

But there's the issue- I can find all kinds of discussion on TBI and conversions, but cannot for the life of me track down a wiring diagram or any info on general function...I'm wanting to know what sensors they used, and if it ever changed throughout the years.

Things I've noticed- no IAT in the air cleaner. Don't even see a plug for it. I think it just has MAP, ECT, and TPS...wouldn't be surprised if the TPS is about the only thing working. Failed emissions with ridiculous hydrocarbon number, despite having no dead misses. Super-black plugs.

I really just need to know if there are sensors missing or not wired properly...an engine wiring diagram would help a lot.

Also, anyone know what's up with the absurdly low-priced HEI distributors on eBay? I mean, Chevy parts are cheap, but god damn...50 bucks for a supposed new distributor with module, cap/rotor, and coil included? How could that even work?

I'm pretty good at tuning Qjets...part of me just wants to grab a cheap aluminum intake and distributor and rebuild a junkyard carb for it. Problem solved...
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Old 11-07-2012, 06:14 AM   #2
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I used to work with the TBI system several years ago and have several parts including performance stuff for them... Sounds like what I have seen before which is the fuel pressure spring has corroded to the point of collapse which is allowing why to much fuel pressure...? Other issue might or could also be the incorrect fuel pump in the the tank has been installed... The all look the same but are not... Also if the temp sensor for the ECU is not hooked up then you stay in warm up mode always which can also cause your issue... Be on later this evening and can help more with it...
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Old 11-07-2012, 01:34 PM   #3
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You're probably onto something...I feel dumb admitting it, but now that I think about it, it does sound like a fuel pressure issue. Possibly that AND a computer control issue.

The injectors are basically spraying a steady stream. And while they're getting some awesome atomization (I'm used to seeing pathetic trickles out of the injectors of these cars), I think it probably is because the fuel is under far too much pressure.

Which would also explain why I had to put a service kit in the throttle body before I could do anything else...it was shooting fuel out of the rear of that main gasket that the raised injector housing bolts to...I guess that would've been pertinent info to give, heh...I'm now one of 'those people.'

Isn't fuel pressure on these supposed to be like 15-20psi or so? I know it's somewhere in the middle between typical port EFI pressures (~30-40) and carb pressures (maybe 5-8 for a qjet?).

Gonna switch gears and go try and read up on the pressure regulation system on these...

Is it fucked up that I long for modern cars (well into OBDII) with complex electrical? Heh. That or mechanical carbs...the stuff in the middle can be a bit funky and hard to work on when it comes to engine control issues, since you've pretty much gotta have the experience with that particular system. These cars, I'm used to valve seals, tune-ups, and fixing minor issues like vacuum leaks. Really, I guess that speaks to their reliability.

My hate has definitely shifted away from GM as of late...more onto Ford. And Korean cars. heh.

Last edited by phucheneh; 11-07-2012 at 01:37 PM.
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Old 11-07-2012, 02:54 PM   #4
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Yup, I am in fact a retard.

I was wondering why when I stuck a kit on this thing, there was no replacement diaphragm for that accumlator. on the back...didn't even realize that was the the regulator in there.

The diaphragm kit is available locally, but I don't see the spring...ugh, what a stupid thing to need to order. But I'm gonna take that apart and check it today.
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Old 11-07-2012, 05:19 PM   #5
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Yep take it apart and you may find the spring is wasted...? The diaphragm is pretty tough and if it were ruptured you would leak fuel everywhere... the nominal pressure is only like 11-15psi if I remember 13psi being optimal... Also like I said if the fuel pump in the tank has been changed be sure its not for a different engine the V6 pump on a V8 (either 305 or 350) would be quite lean but one for a 366 or 427 BBC truck engine would be very rich...

Also there is an adjustable regulator (I have one) that used to be sold...? Its a billet alum design and is really cool to use when working with a modified engine... I made TBI work with a 383 stroker engine with a way out of spec cam (only held 12 in vac at idle spec is 15 in min)... Of course a custom burned chip from Hypertech helped a bunch... Had one of the meanest 1/2 ton ext cab trucks around town for a while... Even had a slight lope at idle when cold...

Back to the problem with yours keep us informed and will get you going... BTW you might have to change plugs already if they are gassed to much...
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Old 11-07-2012, 05:55 PM   #6
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Yeah, it looks like the plugs have been dipped in chocolate and left out in the sun. I think it's a wee bit of oil fouling coupled with a ton of partially-combusted fuel.

There may be some performance parts in this truck's future. It's an original 350 TBI (in the chassis), although I haven't checked the block/head numbers to make sure somebody didn't replace it with a 305. Definitely no 4.3L in its history, though. It's already got some full-length headers; he may wanna stick a cam in it at some point in the future. I told him the heads would be holding him back...still think a carb may be best, since a better flowing intake is cheap, and a properly-tuned spreadbore and can still get pretty respectable mileage. I'm sure I could get more power out of it, even with the smogger heads. But right now the interest is just in not blowing fuel out the exhaust, heh.

I'll look into the adjustable regulator; I saw them when looking for the spring. Looks a lot like a factory one with a bolt sticking out the bottom.

Again, I feel like a 'tard for not catching this. Your explanation makes perfect sense- much appreciated. And it certainly explains why one of the gaskets had a blowout.

edit: oh, and on carbs...one of the biggest complaints is a moot point here; cold starting. I've seen guys run chokeless in the winter. It's not optimal, but you can make it work. A good electric choke, or even just the good ol' divorced choke, does great in the winter.
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Old 11-08-2012, 02:19 PM   #7
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So all I can get locally is the assembled regulator with the top plate that goes over the injectors. Parts are much cheaper online. But I can't figure out the difference between these two kits:

http://www.rockauto.com/catalog/more...154&cc=1031443
http://www.rockauto.com/catalog/more...971&cc=1031443

And why one is twice as much. Searching the part numbers on google, they both seem to be complete kits? I'm wondering if RockAuto didn't fudge their listings...wondering if I should roll the dice and see if I get a complete kit for $20. Worst case, it's just the diaphragm and I still have to get a spring.
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Old 11-08-2012, 02:34 PM   #8
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http://www.standardbrand.com/online%...g/content.aspx

Do the part number search.

Looks like PR131 is higher quality and PR131T is knockoff quality. Both seem to be kits.

PR131 has a much longer warranty.
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Old 11-08-2012, 06:23 PM   #9
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Take note that screw you see in the bottom is a factor preset that is silver soldered in place... Again I would not worry about the diaphragm but wont hurt to change it...

Have you taken it apart as of yet...? The spring should be some what stiff but if corroded pretty bad then it of course loses it tension... But also be sure as I mentioned to check the temp sensor on the intake manifold its for the ECU the one down on the left (driver side) head is for the gauge... If that temp sensor is open or unplugged it will run quite rich too...

Also wanted to know how its doing at idle...? What is it running RPM wise it should be around 650-700...? Do you year the IAC changing air flow...?
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Old 11-08-2012, 06:54 PM   #10
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IAC's gotta be working, or this thing would never run. The throttle plate sits almost if not entirely closed when it's at rest.

Idle is lumpy but around around the right RPM. Can't remember if it had a tach, but I'd guess 600 or so. Feels kind of like a fairly steady miss, but on random cylinders. Like something with barely functional plugs and corroded contacts on the cap/rotor. Or even just like an engine with really bad base timing.

I did the 'electrocute myself a couple times' diagnostic check, of course...meaning I did a 'manual' ignition cutout test...cylinders at the ends of the banks (1 2 7 8) gave some RPM drop, but less than the center four. The outer ones also had more caked up plugs. I'm guessing the center cylinders do more complete combustion because the air and well-atomized fuel comes into them fairly directly; but there's still definitely gas left over

It's fine at high RPM's. Hard to gauge power, but I'd say pretty much in line with common 175-225hp SBC's. As in, the late 80's/early 90's ones rated at 200-250hp...but a bit worn down.

I haven't gotten a chance to look at it again, this one's not at my place. Gonna go ahead and order the cheap $20 regulator. Can't imagine they could purposefully make it that much worse, and it only needs to get it through emissions (sniffer test). Afterwards, probably just switch to a carb...I'm going to keep a lookout for someone ditching a spreadbore manifold and a quadrajet for cheap (after they declare the qjet crap because they don't know how to tune it, and just get a squarebore holley) and maybe try one of those (gotta be) Chinese distributors. Add in a carb kit and some rods...probably $150-200 swap.

God...Chevrolets. I could make so much power with one for freakin' chump change...but I still don't want to.

Oh, and yeah, I'm not sure about the sensor data, what's why I need a diagram. Or just an ECM pinout...I need to access the MIL output so I can see if it flashes some codes...somebody cut a wire somewhere (not at the ECM) or took the bulb out.

Did they ever make one of these without an IAT? I always thought pressure sensors went with air temp sensors (when there's no MAF)...ECT is will only tell it if the engine is cold...nothing about the temp of the actual air charge. There's a 1-wire temp sender on the driver's side cylinder head and a 2-wire sensor near the thermostat housing...I'm wondering where the hell the MAP is...surely I saw one...?

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Old 11-09-2012, 08:25 PM   #11
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No IAT on these or of course MAF either... The MAP should be on the passenger side of the TBI unit and is just an early type but does the job... Also TBI systems will run without the IAC working maybe not to good depending on the position they are can be stuck in but have seen them do strange stuff... Really the only 2 things that will make a TBI over fuel is the coolant temp sensor or the reg spring... I would get the CEL working before anything to get and idea of what the ECU is doing... If the bulb is bad then change it not much to doing so... Then just short pins #1 & #2 with a paperclip and read the codes... (think it was 1 & 2...?)

Trust me when you get it fixed the TBI system is very very reliable and really simple to maintain...
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Old 11-09-2012, 09:38 PM   #12
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I made my life harder on this by trying to work on it like a newer car...where I can retrieve well-known generic trouble codes and read fairly standardized sensor data any time I want. This is not such a thing...need more KISS, for once.

It would appear these are the sensors it's supposed to have-

MAT/IAT - behind throttle in an intake runner or in air cleaner housing
ECT - front of intake by thermostat
MAP - mounted on bracket on intake, vacuum hose to TB (full vac)

TPS and O2 are obvious. And cylinder head temp apparently just works the interior gauge or dummy light.

So...pretty damn simple. I initially missed the MAP sensor because someone had left it not bolted down and I think I wrote it off as a remnant of the removed EGR system...derp. Too used to seeing those damn Ford DPFE's.

But anyhow...this is not running good enough to care about closed loop. And it sees the TPS signal just fine. So you've got your pick of three sensors.

I didn't fudge with the MAP because of above-mentioned retardation. Will check it.

MAT/IAT are MIA. Are you saying those sensors are optional? Do some use IAT (coolest temp), some MAT (warmer temp), and some just go off of ECT (warmest)? If so, wouldn't that mean specific ECM's go with specific engines? If you just plug one of these in place of the other, you're talking about a wide range of temps...on a hot engine, you could see from a little above outside temp (IAT) to 200+ degrees (coolant temp). Manifold temp would fall in the middle.

I do remember disconnecting the ECT seemed to bring the idle up and run a little leaner...so it's got signal. But should an open ECT default to lean or rich? You said rich, right? Perhaps that thermistor is in fact reading very cold but not out of range, so it still runs leaner in failsafe (signal low/high DTC)?
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Old 11-10-2012, 08:36 AM   #13
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If so, wouldn't that mean specific ECM's go with specific engines?
Yes and no... ECU's for 4.3's, 5.0's & 5.7's are basicly the same but some are specific... Explanation is the calibration chips in some can be changed while some cannot (no socket direct solder)... All have the main IC in a socket and you can mix and match... I have ran a V6 ECU with a solder in calibration chip on a 350 (5.7) but changed the main chip (only with a manual trans too)... Also the injectors are different between the 3 engines too, some will say or tell you a 305 (5.0) is the same as a 350 (5.7) but no from GM they have a different color code on top and run lean (or if its a 305 then 350 injectors would run rich)... I have even ran a TBI unit from a 454 (7.4) on a 350 (5.7) by changing the injectors and the use of an adapter that came on 1989/1990 1 ton trucks and using an Edelbrock Performer intake (modified to work with the 15 degree center bolt TBI heads) even setup the EGR and all (fooled it with an oraficed vac can)... Did this on the 383 stroker I mentioned earlier...

As for the MAT/IAT no I do not remember them having these (either)... This ECU relies on that temp sensor (ECT) and if disconneted it should run rich and idle up (cold mode) so if its changing then you are back to fuel pressure...? Are you sure no one has changed anything...? Also must verify what engine you have Vs the TBI unit & ECU/chips... BTW the long tube headers you mentioned will not allow the O2 sensor to work correctly even with a converter installed... They tend to cool the exhaust to much by the time it gets to where the O2 sensor is installed (assume it is...?) for it to react properly if closed loop occurs...? CEL operation is a must will pin point the issue... You do know these do clear all codes when the battery is disconnected for atleast 1 min, not like an OBDII...
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Old 11-10-2012, 11:06 PM   #14
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I know I've seen these with a two wire sensor in the air cleaner housing. It's torward the passenger rear of the bottom place. I don't know what it would be other than a thermistor for air temp?

The last TBI I worked on was an '89 Caprice...valve seals/adjust valves, tune-up parts...the general 'make it not run like ass' package...that car was pretty original and it had the sensor in the stock air cleaner. Plus the little vacuum-operated exhaust stove with the always-missing pipe. Oh, and a valve cover vent from the pass valve cover.

This truck doesn't have anything in the air cleaner. And it's not aftermarket. TB has two studs, which screw into the rear two TB bolts. This arrangement seems odd to me, I'm used to seeing the single offset stud (like above). Also, the PCV goes to a collar that sits between TB and air cleaner. Wonder if this gives any hints as to source of engine...

I'll be sure to look up the engine and head numbers...should help to have a date and know for sure it's not a 305.

The computer in the truck is factory 1995 MY 5.7L.

Headers have bungs and O2's present, but I haven't bothered checking them yet since I know whatever's going on with this truck is keeping it out of closed loop. Or more plainly, even if the O2's were unplugged, it wouldn't run nearly this badly. Are they not heated O2's?

I would assume they would be fine in the header collectors, but I can see how the signal from that sensor would surely differ from one stuck in a log manifold. What about welding an O2 bung into a single header pipe? Wouldn't be optimal, but would the voltage be more in the expected range?

If this thing would run properly, it could probably pass a sniffer test in open loop. But it really runs like ass. I'm gonna actually have to time to check it out until I find the problem tomorrow, so we'll see what's up with it. Part of what needs to be addressed is highly carbon-fouled plugs and exhaust backpressure (dual exhaust, one side feels weak- think the cat is plugging)...but both will only get worse if the ECM doesn't meter fuel right.

I had a funny thought earlier about how this thing could probably run E85...it probably gets enough extra fuel that you could find stoich somewhere in the 60-75% ethanol range. What a way to 'cheat' emissions with an unintentionally modified flex fuel truck.
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Old 11-10-2012, 11:18 PM   #15
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Oh, and the reason I was thinking of E85....the person with that Caprice contacted me.

Him: It failed emissions. HC 240. (limit is 220ppm)
Me: How much gas is in it?
Him: Quarter tank, maybe.
Me: Find a station with E85, pump a gallon in it, and retest.
Him: ...68ppm
Me:

E85 is worthless as a fuel...yes, I know, higher effective octane and whatnot...yet even boosted, high compression cars can't seem to make up the power loss...Better off just turning the boost down and running 93. But it's the shit for worn out engines trying to pass a sniffer. No more buying denatured alcohol from Lowes.
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Old 11-11-2012, 11:27 AM   #16
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No more buying denatured alcohol from Lowes.
LOL... Should only have a single O2 sensor and dont remember for sure but most were only 1 wire not heated but 1995 was a strange change over year too so could be a heated type...? Also as for an air temp sensor the trucks did not have one but possibly the cars did...? Do not remember on them either...? Just look on the block deck front right (passenger side) and get the engine code should tell you what it is, no need to check the heads unless some one though putting 305 heads on a 350 would help with compression...? Dumb stuff to do so the chambers shroud the cylinder bores so severe flow issues DOH...!

Did realize this is a 1995 which is the same as a 1994 and the ECU's are different from the earlier models... These have the single long chip and the calibration is built on to it not on the ECU board like in the earlier models... I'm still back to a fuel pressure test which you can remove the filter under the drivers seat area in the frame rail and put the dummy log in and hook a gauge to... Need to know what it is...

Also this must be an early 1995 model since the late 1995's were newer vortec designs with the good flowing heads and a totally different fuel injection system...
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