Hello, just put my rebuilt 350 back in and after a multitude of problems, I get it running, get it timed in right, finish plugging everything in and I get a code 33. Following the troubleshooting chart I got from Trickster some time ago, I got under 4v at Pin "C" at the MAF plug, Checked between pin "C" and B12 on the ECM harness and read 0.00 ohms, not infinity, at the Pin B12 on the ECM, I should get a +5v reading right? I got 0V there and 12v on Pin B1, which according to the chart, I have a bad ECM. Im just wanting to see if theres anyway I can doublecheck this, or am I right in my measurements? I used a body ground for the ECM check if it matters any. Thanks for any help!

 

Anyone? Just wanting to make sure I made my checks and diagnosed the correct part being bad before I got order a new ECM tomorrow, thanks!

 

As a general hint, DTC '33' is frequently caused by improper PROM calibration and/or an improperly adjusted TPS.

Are you certain you have the terminal assignments correct?



You should get well under 4V at MAF connection 'C'. It should be an analog voltage signal between 0-5VDC and should be more like 875mV at idle.

Your resistance check between MAF connection 'C' and ECM terminal 'B12" has only proven that the wiring is intact. No problem there.

If you want to read 5V at ECM terminal 'B12", you'll have to find an open stretch of highway (or track), probe the terminals, and floor the accelerator until the engine reaches about 4,500 RPM. That should be close to maxing out the MAF output. The normal signal there should be that same 0-5VDC analog output from the ECM.

When you pull out your "bad" ECM, send it to me. I can always use a spare. I'll pay postage.

The ECM ground, MAF analog signal ground, and chassis ground should theoretically be at the same potential. However, age, weather, and motion can create resistive connections and alter that state. It would be a good idea to clean up all the grounds you find in any case. A poor analog ground at the TPS can also result in that low TPS signal that would contribute to or induce the '33' in the first place. In the interim, use the analog signal ground at the MAF connector for your reference.

 

Thanks for the response, brings up another question on my end though. I was adjusting the TPS voltage and the highest I could get was around 3.3 VDC, which is lower than its suppose to be correct? So should I be troubleshooting a ground along the TPS line as my possible fault? What happens is when I plug the MAF in and start the car, it revs to 3k RPM and shuts off every time. I unplug it and it runs just fine. Thanks for the help, hopefully I can figure this out quick!

 

Just for giggles, measure the reference voltage across the TPS 'A' and 'C' terminals. You should have 5.0VDC, or within 0.1V.

*If the 5VDC across the TPS is too low, you need to check connections all the way back to the ECM.

*If you read 5.0V across the TPS, reference an engine ground and see if the reading changes. If so, you have a ground problem. If the reading is still the same, move on.

*If the reference voltage at the TPS is good, check the 5V source from the TPS and reference the ground to both the MAF 'A' and 'B' terminals. If either or both result in less than what you read at the TPS, you have a problem with grounding to the MAF.

Good luck. You're getting closer.

 

"A" to "C" read 5.00VDC
Read 5.0 VDC from "A" or "C"(cant remember right offhand) to the ground on the back of the heads, and read 5.0V from the MAF connections "A" and "B" to the TPS.

Dunno what this means, or if it helps, but thanks for your help.

 

What that means is that the ground connections are intact. That's one less possible problem. That also means the logic power supply from the ECM is working properly.

Try measuring the TPS output voltage at the closed throttle position. That would be between the 'B' and 'C' terminals of the TPS (the Gray and Blue wires) with the ignition ON. There should be between 0.48 and 0.58 VDC in that state. If not, adjust the TPS position to obtain as close to 0.54VDC as practical.

If you still suspect the MAF, measure the voltage between MAF connection 'B' and 'C" with the MAF ductwork connected and engine idling. There should be a reading of around 900mV.

If you have an altered MAF (no screens, no heat sinks) the readings may be skewed by the uncontrolled air path through the inner sensor array, and you'll be shopping for a new MAF.

 

The highest I can adjust the TPS to is 0.34 VDC. I was unable to check the MAF plug at idle yet (Don't want to wake everyone up hehe). The MAF sensor itself was just replaced last august and It didn't get much use before we took the car off the road for the engine rebuild.

 

Ok, I went and dug into it some more. With the Ignition on, but the car not started, I checked at the blue and grey wires and got 4.61VDC. From the blue to black wires I got 0.36VDC, from the Black to Grey wires I got 4.99 VDC. Hoping this helps some, your a huge help thats for sure Vader, thanks!

 

BINGO!

Low TPS voltage. If you manually open the throttle while testing the TPS voltage as you describe, does the voltage increase?

You really need to get that up to 0.48VDC or higher to prevent the '33' errors . Like I mentioned before, a '33' is very frequently the result of a misadjusted TPS, and on the low side of correct. Finding out why your's is too low may be a problem all by itself. If you have an aftermarket TB, the TPs lever may not be in the right position. You may have a TPS that has a lever that slipped or bent, or may have been given the wrong TPS (holes in teh wrong locations). If you can alter the linkage to get the voltage to the correct level at idle, the error codes should be gone. You may want to check the minimum air position of the throttle plates before getting too carried away with final TPS adjustment.

 

Heh. I remember adjusting the minimum air at the TB last summer when I started having these problems, so I adjust it up and got the TPS to 0.53VDC. STILL having the same problem though, no codes are being thrown out, but when the MAF is plugged in and I start the car, it either barely idles and shuts off, or idles up to 3k and shuts off. If I start it without the MAF it runs fine. Im pretty much stuck as far as what to do about it at this point. Thanks for the help on the TPS, maybe you can point me in the right direction on this one lol.

 

Heres an odd question, could lack of vacuum cause the car to shut off when I plug the MAF sensor in? I drove the car across the yard to park it last week and I had to stand on the brakes to get it to stop, there wasn't any vacuum there, so I ordered a Vacuum canister from Summit. Got it today, but gotta get some bolts and vacuum hose tomorrow. Just an off the wall thought. Thanks!

 

Instead of a cannister, find out why there is no idle vacuum. Is it a mostly stock setup?

 

Errr, well, heres what I have had done to the engine.
Block was bored .030 over, forged pistons/rods, crankshaft polished/turned, engine balanced, LT4 Hotcam, new oil pump, new bearings, Heads got a tri angle valve job, new springs and valves, 1.6 roller rockers. Hedman long tube headers, 8mm wires, accell shorty spark plugs, new water pump, TPI intake system is stock, everything else is pretty much stock.

 

Ok, installed the vacuum canister today, ran some new vacuum lines where I had patched them before. The car started right up and revved to like 4k RPMs. So I put it in neutral, started it, put it in drive and let it idle for about 5 minutes. After this it started acting fine, idling at 1200 or so in park and about 800 in drive. The brakes are still a bit hard to get the car to stop, maybe rusty rotors from the car sitting without rims for a long time. It drove fine, but when I was ata stop and in gear, the car would surge forward and idle up and down, it was fine while driving. Smells a bit rich, and im getting the code 33 again. So it seems its better than what it was, but still not perfect.

 

Drove it all the way home a bit ago. When I started it up, it revved up real high and dropped back down to around 1200, It drove fine, the brakes are still real hard, got down the road aways and turned into my driveway and I had the brakes all the way to the floor and the car revved up and wanted to keep going, the brakes barely held it, I had to put it in park and restart it to get it to the house. Just trying to throw all the symptoms out there, really trying to get it runnin right

 

Question about the MAF line. In the diagram it shows that Pin "A" and "B" are ground, Pin "C" is the 5v ECM reference, Pin "D" is the burn off, and Pin "E" is the 12V. Now if the key is turned forward I should get 12V at Pin "E" right? I checked and I get ground at A and B and 5v at C, but I do not get 12v at Pin E. Could this be my code 33 problem?

 

You should only get 12V at pin E after the engine is turned off and after the oil pressure switch opens (when the ECM initiates a burn off cycle).

You should not read 5V at pin C unless the MAF flow is maxxed out (or you're measuring P-P voltage on an FM MAF).

 

So this could be the problem? Im measuring from the plug with the ignition turned on, the car is not started. Im hoping I fixed my vac leak, apparently I had used the wrong intake gaskets and they were wet wet wet. I also replaced all the vac lines in the engine compartment. Other than that, I have no idea where I could be getting a vacuum leak that large, and then I have the MAF problem hehe.

 

Not meaning to hijack the thread, but I am having similar problems- after having installed the third MAF sensor in the car, replaced relays, redone loose connectors, etc.

When the car is at idle and the MAF is unplugged, I get 4.95 volts from pin c to ground, measuring the engine side of the plug, of course. I was checking for the 5 volt signal from the ECM.

Is this correct?

 

Anyone? Hoping to fix my spark problem today and see if my Vac leak is gone, hopefully it is, then Ill just have to deal with the MAF. Oh, and an off question, a single wire 02 sensor ok for my engine setup on longtube headers? Or would I need to switch to a heated 3 wire? Thanks!

 

Got it started up again today, backed it up and the brakes felt fine, havent gotten to drive it yet tho, hard to do with my son around lol. I still get the code 33 and now im getting a code 42 also. Beats me whats going on, help if you can! Thanks!

 

NM im an idiot this morning hehe. Had the EST disconnected still. Plugged it in, cleared the codes and now im getting a code 34 instead of 33. Oh the confusion!!

 

*New development* Ok, so I unhooked my Vacuum canister and the fault goes away, I drive it and the fault does not come back, but I dont have enough vacuum for my brakes, hard as hell to stop. I hook the Vacuum canister back up and I have very good brakes, car drives very nice, but I get the code 34, which the book says Low voltage, high vacuum. Is there anyway to fix this fault and retain my vacuum? Thanks!

 

Anyone? Really wanna get this taken care of. Thanks!

 

Heh. Now I get a code 33 and code 34, seems weird. Hopefully I can figure this out.

 

i'd seriously start looking into the vacuum problem

i bet its the root of all evil here

 

Well, when I replaced my manifold gaskets with the correct ones, my major vacuum leak went away, and now with the vac canister, I have good vacuum, but both faults. Without the vac canister, I have no faults but my vacuum sucks. Ive replaced all the vac lines save for the line coming out of the firewall into the T connector at the rear of the plenum, and the 2 big vac lines coming out of the charcoal canister. Ive heard tale that if the charcoal canister is bad, it can create bad vacuum? If I take the hose off right after the car is run and shut off, and I smell gas, it is bad? I really can't think of anywhere else I could have a vacuum leak, and Id really love to find it and be rid of this bug hehe. Thanks!

 

Now im back to just the code 33 and just as lost as ever. I feel like im just shooting in the dark, which I dont really like doing. What else could possibly cause a vacuum leak?

 

Anyone?

 

Check the plumbing of the vacuum reservoir. There should be a checl valve right in front of the vacuum reservoir (just like the one that should be on the brake booster reservoir).

Pardon the crude diagram, but it should look a little like this:

 

I put a hose clamp on the hose going into the check valve going into Vacuum canister(im assuming this is the resevior your talking about, I dont have one other than that.) and the code 34 dissappeared. Unless the check valve isnt seating well into the canister, ill check this again. Thanks!

 

Eh, so I replaced the vacuum lines at the charcoal canister (me being dumb today, not sure which line goes on the top or bottom, so the top one is going to the TB and the bottom going to the gas line portion). The fault didnt come back while the car was idling, so I took it out for a drive and after about a mile or 2 the fault came back. So I check it at my bro's house and now im getting a code 34, no code 33 at all. So I drive it back home and get it up to about 55-60 and it just shuts off, I pull over and it cranks right back up and I get it home. Brakes are still a tad funky(hard to stop a bit) when im creeping, but still work good, and work fine while im driving. Even after I got the code 34 and went home, the fault didn't reappear until about a mile down the road. At this point I just dont know what else could be wrong, help if you can. Thanks!

 

There should be a VECI label affixed to the radiator shell or underside of the hood which shows the correct vacuum hose routing.

Make sure the MAF is sealed to the intake ductwork.

Inspect the MAF and intake ductwork for splits and cracks.

Set the TPS to 0.54V at the warm idle position.

Remove and cap/plug all the vacuum hoses and connect one at a time to try to isolate any potential leaks.

I'm still suspicious of the brake booster and reservior check valves. What is the idle vacuum?

And just for reference, the "book" isn't quite clear enough in the descriptions of error codes. A '33' is for abnormally high indicated MAF flow, and a '34' is for low MAF flow.

And just to be certain, in all that modding, is the CCV tube still connected to the RH valve cover and TB fitting, or is there a different crankcase vent, like a breather type oil filler cap installed? Is the PCV new or cleaned? With the LT4 cam's longer duration, you may need to replace the PCV with a valve designed for a large V-6 instead of the stock unit. The flow/vacuum calibration is quite different.

 

Interesting and thanks a ton for the info! First off, the, umm, clasp?(metal band) that goes around the end of the MAF, mine isnt on as tight as it used to be, you can spin the MAF around, bad yes?

As far as Idle vacuum goes, according to the canister guage, it likes to sit around 11 after I hit the brakes, it will go to well over 30 if I rev the engine and sit there until I hit the brakes.

The same CCV and PCV lines are still being used, I replaced the grommets on the valve covers. The CCB line going into the Plenum is easy to take off, almost slips off even, should this be alot tighter?

As far as the rest of it, Ill get out there ASAP and get to work, thanks a ton, I honestly dont know what id do with ya. Pay alot of money to a shop I guess lol!

 

Ok, by warm idle TPS voltage, Its around .52, I moved it up to .54 but the car started idling irratic and revving up on its own, so I lowered it back down and it idles ok for about 5 minutes then idles real low and shuts off.

All the ductwork is ok, I couldnt find any holes or cracks, the intake portion of the ductwork that goes to the MAF has the clamp I was referring too, its loose, don't know if that makes it not sealed though.

I called advanced and the only V-6 they had was a 2.8 and the PCV valve is cheap so Im gonna pick it up in the morning.
Thanks again!

 

The MAF may turn with a little force, but should be relatively snug. If it just spins easily, you may have a problem with leakage. I've seen some with the band clamps that are crimped, and some with the common type hose clamps. You may want a 4" hose clamp to seal your's better.

The guage on the canister is only indicating stored vacuum. A vacuum guage connected to the plenum would be a much beter indication.

"CCB" line? If you meant to write "CCV" line, it should NOT be connected to the plenum. It should connect to the upper hose nipple on the right side of the throttle body. It, too, should be able to be removed, but should have a relatively snug fit.

 

Sorry bout that, I got em backwards. Im getting a V-6 PCV Valve today and ill throw that on and see how it goes. I started it up today and it ran fine, idle was about 1300, actually a bit more steady than before, maybe because I raised the TPS voltage a bit. The fault still came back after it ran for awhile, still the code 34. Im going to get some carb cleaner and spray all my vacuum line connections, I was told that would help me find a vacuum leak if I have one. Thanks again for the info.

 

A high TPS voltage will contribute to a '34'.

The ECM uses only a couple basic inputs to calculate the allowable MAF flow range:

• Engine RPM (reported by the HEI module via distributor reference pulses;
• Throttle opening (reported by the TPS);

At a given RPM, the position of the throttle will determine the approximate volume of air that should be flowing through the MAF. If the engine RPM is being reported higher than actual (not likely due to the simplicty of the system) the ECM will expect more intake air flow at any given throttle position. Conversely, at any given RPM, if the throttle is actually open less than the TPS is reporting, less air will be flowing than the ECM expects. Either of those will set a '34', even if the MAF is measuring and reporting the exact amount of air that is flowing through it. Therefore, setting the TPS higher will tend to trigger a '34', as well as contribute to a rich mixture. The CALPAK contains tables that define allowable MAF flows based on these inputs, so it "knows" what should and shouldn't be correct at given RPMs and loads.

Setting the TPS too low can also cause lean operation and contribute to a '33' as a result of the same ECM calculations.

Set the minimum air position close to the specification if you can. Then set the TPS to the specified voltage (0.48-0.58VDC), or even a bit toward the lower end of the range. At least you will know that those variables are in range, so you can continue the diagnosis.

 

I currently have the TPS set at .052 VDC, which the car seems to like, if thats possible lol. The only thing that kinda irks me a bit is how I was getting both the code 33 and code 34 at times, seems odd.

 

When I first got my 305 TBI, it would go from a somewhat OK idle and out of nowhere jump up 1000 or 1500 RPM for some indeterminate period of time & then fall back to normal. I had a bad TPS. It's worth a shot. Grab one and pop it on long enough to see if the problem goes away and just return it if the current is OK.

 

the TPS is actually relatively new, it was just put on before the engine was ripped out.

Question though, the hard line that connects the PCV valve to the manifold, I went to put the V 6 valve in and I pulled it off of the Manifold and it broke, cracked in a few places, do I need to have another hard line like this, or can I use fuel line? thats what I put on there anyways, just had it laying around. It idled a bit higher with the V6 valve in, I put the old one back in and it idled low and I think it was going to die on me. Just concerned about the hard line at the moment, wanna make sure I have it right, and the PCV valve that was in there has a L shaped connector on the end, but I can't get it off to save my life, so the old hard line wouldnt fit right over it. Can I get one of these also?

The 4" clamp didnt work, the MAF moved way too much inside it, so I went back to the original clamp and it holds good, MAF doesnt move much.

 

Ohboy, finally got the new PCV hose, had to order from Chevy, put it on with the V6 PCV valve and the car would start and die, switched the PCV back to the V8 and did the same thing, unplugged the MAF sensor and it starts and stays running. Man im confused

 

For you MAF guys: Could this be a faulty MAF, possibly caused by an inoperational burn-off circuit?

I don't know about how the MAF circuit works with the PCM (from a "limp-home" standpoint), but when my MAP sensor died (cut vacuum line), the car cranked over VERY roughly for about 4 seconds and then the ECM threw a code and it ran just fine (but with the SES light on...Basically, my ECM saw the sensor was dead and ignored it; going into limp-home mode.). Does the PCM for a MAF engine just freak out when the sensor's bad or does this look more like a wiring problem?

 

More confusion! I swapped each relay out with another and ran it, seemed fine, I cleared my codes and restarted, it idles up to 2000RPMs like I have my foot on the gas and keeps goin up! when I unplug the Maf sensor it idles normally, I have NO idea what the problem is!

 

When you cleared the error codes, you likely also dumped any BLM memory. The ECM will have to get about 20 minutes of various range driving to start to write new tables. The high idle is common on a cold-started TPI, then should drop gradually as the coolant temperature increases and internal timers are satisfied.

Most importantly, were there any error codes present after you cleared the ECM and first ran the engine? I'm certain you generated an error when unplugging the MAF, but were there any codes present PRIOR to unplugging it?

 

No codes whatsoever once I cleared the ECM and started the car with the MAF plugged in. It was kind of freaking me out that it would keep idling up like I had my foot on the gas, should I just go with it? I only got the code 33 once the MAF was unplugged and no other faults.

 

The ECM will set and lock a MAF error rather quickly. If the MAF reported flow is out of range for more than the 600mS internal timer, a code will set. That is supposedly just long enough to ignore MAF reporting lag from large throttle transitions. If you had no error codes, I'd go with it.

If the idle doesn't start to drop within 90-120 seconds, you may have a problem with a sticky IAC, vacuum leak, etcetera, but not likely the MAF.

 

Eh, went out after all this rain stopped and started again with the MAF plugged in, didnt idle high, idled around 1200 RPM in park. Took about 5 seconds for a code 33 to pop up, I cleared it once and it came right back. Where this puts me, I have no clue lol.

 

A '33' is very often the result of a TPS that is set too high. That might also explain the higher initial idle RPM.