5.7 TBI rough start help

[WayneJ]

Hey guys, having an issue with my 1994 350 tbi. This is an issue that has been going on for a few months (closer to a year) and has been getting worse and worse very very slowly.

If the truck has not been ran for an exetended period of time OR has been allowed to go completely cold than it will start, but stumble, misfire, sputter out, and almost die. UNLESS I throttle it up to 2k rpm or higher. At which point it will idle back down and run a normal procedure of a high idle when cold then taper down to the normal idle range. When it first started doing this last year it would lope after startup, then smooth out. Also since its been below 20 degrees here in the great state of Alabama it has gotten much worse. There is also a very noticeable loss in power when the engine is not yet up to operating temp. In the past year I have replaced the following: IAC, TPS, Fuel pump and filter, intake manifold, Entire distributor, EGR, O2, CTS, MAP, Knock,multiple vacuum checks, a few connectors, had an electrical fire when the battery cable to the starter welded itself to my exhaust (what fun that was), and had my valve seals replaced. The only things I have not tried are the fuel pressure reg, and the computer. The previous owner of the new engine did a throttle body rebuild. The injector spray looks good, although sometimes I do notice that good old rich mix smell, but it could just be the lack of a CAT.
Very rarely the truck will not start at all no matter how much I crank it, unless under WOT, at which case it will fire right up like nothing is wrong, and it only does this when cold.

The truck has 446k on the body, but the motor and trans have about 85k from another 94 k1500.
I was throwing a code but the O2 sensor and knock sensor were bad, since I have replaced them I have not had a code.

Open to any and all suggestions!!

 

[DerekTheGreat]

Hrm. I If it were me, I'd verify base idle. Whenever anyone plays with the TBI unit there's a chance that they adjusted it or it's no longer right due to whatever they did. That's how my C3500 454 TBI was. It would start but the idle would surge up & down unless you held the throttle constant and it warmed up. I looked on forums such as this one and people mentioned base idle and to check for the plug that covers the adjustment torx screw. Sure enough, it was punched out. I decided to rip the whole TB off and do a half rebuild, new O-rings, gaskets & regulator. When the injectors came out, I noticed gunk in the screens of one of them but the fuel spray pattern was fine... Also, the gasket the PO used for the TB was half way in the intake, it was some home brew "cut it to fit" thing that the guy must've done while drunk... Anyway, I slapped the TB back on, followed the procedure to bottom out the IAC valve and hooked a scan tool up to the truck to see what the RPM was. It was too low, like 400 something, so I bumped it up to where it should be (~600 IIRC) and after that, the truck fired up beautifully with a smooth idle after that.

I highly recommend a scan tool though, this way when you go to start it up you can monitor the input from the sensors. Mine has most recently helped me diagnose a bad TPS on my '89. It never threw a code, but the idle was all over the place so I thought I had IAC or a leak somewhere. 10-15 minutes of thumb twiddling fun watching the data and the sensor revealed it was erratically changing from 0.58v to as high as 0.78. And when it did that, IAC counts would go up. "There's your problem, lady!" Replaced TPS with junkyard unit and she's been peachy since.

 

[redfishsc]

Be sure the coolant temp sensor you replaced is the one that the ecm reads, and not the one the dash uses. I think the TBI is similar to the Vortec and uses a different sensor than the dash gauge.

Mine was faulty on my 99 5.7l, would not read above 170ish

 

[WayneJ]

@redfishsc I replaced the the correct one, (one by the thermostat housing).
@DerekTheGreat Do you have a scan tool you would recommend? Ive always just used the paperclip to read codes, but I know its no where near as informational. What does IIRC stand for. In gear it idles right at 600, but I will check the TBI gaskets. Do you know how to adjust RPM on the Throttle body?

 

[DerekTheGreat]

There's a guy on another thread who posted something about his mechanic saying both sensors need to read within 10 degrees of one another but off hand with all the GM literature I've read & wiring diagrams I've looked at, I don't think that's true. Dash gage is certainly important though. When temps were sub zero around here about a week ago my truck's thermostat would not open despite being a brand new Stant unit and the system having been flushed & then refilled with fresh green stuff. So, that gage saved me from burning up a valve or worse.

I've only used the one scan tool after scouring websites for a good/cool one, the Snap-On MT-2500, aka "The Brick." This was recommended by peeps from ThirdGen.org to handle all of your OBDI duties. I liked the look of it so bought it. Be sure to get one that comes with everything, such as all the cartridges for various makes & model years up through at least 1998, the cables and the case. I got mine for $300 of eBay, so depending on your funds that may be a deal breaker but can pay for itself in time if you use it. Not sure what they're going for now as I've had mine a little over a year now.. As for the device itself, the display kind of sucks, because you only get four lines of data but can scroll through using the wheel rather quickly to see what everything is doing. You can also customize what the lines display, as in "Freeze" certain data points like the o2 sensor so it's always displayed.. It remembers the last car you input so long as the 9v battery inside it is good. Lights up (including the display) in darkness so that's pretty nice. It has troubleshooting tips for any codes you read an allows you to do diagnostic stuff such as AIR testing. Differs for every car & manufacturer. I have old Ford EEC-IV stuff and most of it made before 1992 doesn't tell you anything, except for codes or functional tests like mentioned earlier. Stupid Ford crap and their "Break-Out Box" and dumb EEC-IV connector being under the hood instead of under the dash... But, at least The Brick provides enough cable to go from under the hood to into the car should you encounter stuff like what Ford has. For road test stuff it can be crazy to try and read + scroll through data without smashing the car so I suggest a second person come along and read off stuff to you. There are many others out there and perhaps better ones that are smaller and easier to use so be sure to take my suggestion with a grain of salt. Also, The Brick claims to be able to work with OBDII stuff but I don't own anything that's OBDII so I've never tested it. Has the connectors though, but my domestic cartridges only go up to 1998.. Speaking of the cartridges, mine came with a bunch but it seems many were available separately and the later ones superseded & phased out the older ones. For example, I've got separate Ford, Dodge/Chrysler/GM cartridges but then also have another set of cartridges (Thing has two slots for them) which include all domestics through 1985-1998.. GF bought me a Blue Driver for Christmas but that's OBDII only. Blue tooth so it seems to be cool as it sends all the crap to your phone so aside from the small unit that plugs into the diagnostic port there's nothing else to carry or store. Keep forgetting to ask friends to let me try it out on their stuff.

IIRC stands for "If I Recall Correctly."

Right, once my old '88 warmed up it would idle at 800 rip 'ems like it was supposed to.. It's the truck that prompted me to buy a scanner since it was such a ******* haha. The IAC valve is pretty flexible and this early stuff really doesn't care what position the sensor is in so long as it can obtain the result it's looking for, like 800 rpm or whatever it wants. So, after I adjusted my base idle I noticed that the counts when warmed up were lower than they used to be, which means the IAC valve didn't need to open as much to get the idle where it wanted it to be. The GM fuel injection books I have describe a range of what the counts should be given the model, engine & trans. Of course there's other ranges too but that's another topic. So for my '89 I think the counts at idle are supposed to be anywhere from 0-25 IIRC, I've seen anywhere from 0-16 at hot idle.

So the procedure to set the base idle is as follows. Keep in mind that you want to check to see what base idle is before fiddling with that screw, yours could be fine. You'll also need a reliable tach or scan tool. The Tach in my truck (I swapped the moonie thing for a cluster which looks like yours) is off by at least 100rpm so it's not reliable.

Procedure as ripped from a random website (It still runs):
"Remove plug over the idle screw located on the right front of the throttle body by using a hammer to drive a punch into the plug and then using the punch to pry the plug out of the throttle body. Discard the plug. Start the engine and let it run until it warms up.

Turn off the engine and leave the ignition key in the "Off" position. Unfold a paper clip and bend it into a "U" shape. Push the ends of the paper clip into the "A" and "B" terminals of the ALDL (Assembly Line Diagnostic Link) connector, also called the diagnostic connector. In GM vehicles that use this TBI injection, the connector is a black plastic rectangular connector located under the driver side of the dash. The "A" and "B" terminals are located on the top right of the connector." To this I'll add: Once you do this, flip the key to the "run" position for about 30 seconds. This will cause the IAC motor to close as fully as it can. Go unplug the IAC motor, flip the key to "off", remove that paper clip grounding out terminals A&B and then start the engine. nsert a T-30 or 25 Torx driver into the Idle adjustment screw that is located behind the plug you may have removed earlier. Turn the screw to adjust the idle speed to 600 RPM +/- 50 RPM. That's a guess as to what your truck should be. Shop manual for mine says that's what base should be. I assume your truck is an automatic 2WD? I'll see what it says for that. Once the idle is adjusted, turn the truck off, plug the IAC motor back in and you should be good to go. There's an easy procedure to reset the IAC motor (should you need to do that) but I forget exactly what the order is. Something about flipping the key on or off for 10 seconds... You shouldn't need to do that though.

 

[JCribb]

@DerekTheGreat yeah that was me, but all I have is that guys word, and he isn't my personal mechanic, so I have no proof of who is correct, for all I know that guy could be blowing smoke up you know where.


What I do have is that I replaced both of my sensors with OEM parts, and my truck runs 100 times better than it ever has in the last year.

 

[Ken K]

First, without a scan tool, things get difficult to go straight to the root cause. EGR control was an electric solenoid and they would freeze a little condensation. This condition holds the EGR open at idle. If yours has an electric EGR, wires & connector (Not familiar with your model), The use a linear pot to provide position feedback. This would set a code. A warm up cycle is part of OBD-II. It means the ETC & Intake air are within 10 degrees F of each other, that is a cold start. Once ETC hits 176 degrees F, that is 1 warmup cycle. Type B codes clear after 40 warmup cycles. Type A clear after 80. Runs test a passes, CEL goes off, but history is saved. Type B code are 40 test ran / passed while type A are 80. Type A codes are emission no-no's. CEL flashing 1 time per second is cat danger. Type B codes are not emission, but device / circuit codes. Example, you left something in-plugged after a repair. Some codes run off clock time, like fuel tank vapor leaks. They run every 8 hours of operation at temp. P1870, internal slippage in automatic trans (Input shaft speed - v - Output shaft speed) the engine has to be warm, running a highway speeds for 6 -7 minutes. It is called the enable criteria. Certain codes not set, if so, they abort certain test. The paper clip is useful, but flashing codes 3 times, lowest first, highest last. Check for vacuum leaks, in-plug EGR and try for a few days (Winter in Georgia / Try Omaha - high of 3 degrees F today), but having proper TPS, use DMM, 0.4v at idle to 4.70v at WOT. Move slowly and look of lost or drop-out voltages. I prefer lab-scope for that one. MAP, TPS grounds are dedicated to ECM. Thru ECM to G100, thru engine back to ground cable & battery. Run engine, "TEE" pin back probe and reading on DMM should be less than 0.400 volts. This is OEM max which is too high. I would expect to see 0.050v or 50 millivolts would be better. This is a voltage drop, with circuit on, current flowing...and checks entire circuit. Note: Battery in bad shape? Weird internal battery issues cause ECM problems. Alternator? Yea, diodes rectify A/C voltage, but some A/C voltage leak thru. DMM set to A/C, start engine...if checking at alternator, rev up engine & DMM reading should not exceed 0.090 vAC. Normal is ? Depends on diode condition, type, brand, 6 or 9 total as I have seen a max of 0.012 at 2k RPM. A/C signals exist on DC wire circuit at the same time. They mess with serial data which is 0v - 7v on / off like class 2. The battery acts like a condenser and calm the A/C signal so reduce by a factor of 3. So, at alternator, your reading 0.070vAC, battery reads 0.022vAC. Hoppe this helps the problem...if not one some other day. It's a good way to check alternators also. Obviously, this issue cause more problems on vehicles after 1992 as they have several types of serial data modules need to function. GM LAN or (CAN-controller area network) start in the Vette in 2004. Now in every thing GM on DLC pins 6 & 14. CANdi module needed for TECH2 at that point.

 

[Sonny Baker]

I see you replaced the distributor. Did you replace the Ignition control module ?

I just went through this exact same thing with my 91 TBI and replaced all the parts you have ( except the distributor) and finally fixed it when I replaced the ICM.
Truck ran great on a cold start above about 50 degrees ambient temperature.
Below that it popped and missed for about 30 second to a minute, then cleared up. Restart was fine unless you let it cool back off and it would do the same thing again.

Drove me freaking nuts !!

 

[Sonny Baker]

Thought of something.
Unhook the timing advance wire before a cold start and see if it runs smooth but sluggish.
Mine would, and that led me to believe the timing was being overadvanced.

Sonny