At least I think its ECM issues. TIming is all over the place, mixture swings wildly from rich to lean, engine bogs so bad I can't drive the car. Checked fuel pressure, 14 psi constant. New DUI distributor, MSD wires and AC Delco plugs, checked balance on TBs they are spot on, base timing set with EST unplugged to 8*. NO CEl codes, timing pulled with knock sensor connected or disconnected, you can watch the timing fluctuate at idle with a timing light, blipping the throtte makes it advance and retard like crazy. I'm at a loss as to what to do. New ECM? Voltages are good everywhere. I hope DUB sees this post and has seen this before. I'm looking at going to a programmable ECM (EBL Lockers) but I don't want to drop the cash if I have other issues. Engine runs quiet, no knocks or pings, no exhaust leaks, no vacuum leaks.

 

have you tried spraying carburetor cleaner around base of TB and intake to check for vacuum leaks?

 

Indeed I have. No leaks at the tb's or the lid of the manifold.

 

You may try cleaning and checking your ground connections. I had similar problems once with ground at engine block. Stupid things having a senior moment cant remember the name doodads on the TB's that control air flow can mess you up also. I have a spare ecm you can try .

 

Sorry to read your having problems Drawmain. U have such a awesome build going.

Well fluctuating timing is 1 of 3 items.
1 - distributor
2 - dampner is slipping making a false timing change while timing is still good and problem is somewhere else
3 - ECM is having problems controlling timing

No DTC and engine running wild means ECM is doing poor job.
U could check the O2 output as it sound be 0 to 900mV with closed loop stoichiometric 14.7 a/f reading or trying to control near 450mV.
O2 indirectly controls the fuel trims so if mixture swings wildly i would look a the O2's output.

Pull the dist look it over for loose or frozen parts - hey even new dist can be goofed and get through quality control. I dont think your dist has weights though as the ECM controls advance i think. Check dist shaft play. Look over dist cap and rotor - ya know its easy to clip on the cap at the wrong position.

BTW what did u do for a ECM/chip tune with the new build - heads, cam, intake? Your engine vac maybe much lower now than stock and the ECM maybe correcting. Im thinking a speed density system here with absolute pressure used for MAP/vacuum telling ECM what engine load is. Dont know what your ECM VE map looks like or is but your VE has definitely changed.

 

Been there, done that with the connections. Good voltage and connex everywhere. I may take you up on the ECM offer, thanks for the idea on things to check!

 

1. Dist is new, DUI no loose or frozen parts, problem existed with original dist, pulled it open to look and found broken plastic, dry cracked wiring which lead me to the DUI dist.

2. Balancer was replaced last year, double checked mark against tdc when rebuilding, it was spot on. Double checked again when issues started, still spot on.

3. ECM is what I'm thinking. I've replaced everything else. Balanced TBs, re installed intake (Renegade), fuel pressure confirmed at 14 psi even under load. Definitely running rich, you can smell it, running a three wire O2 sensor that will get replaced soon to eliminate it as a cause.

Dist is ECM controlled, no weights. All wiring good, no undue shaft play, cap is on correctly and locked down, timing set at 8* with EST unplugged. Timing varies wildly with knock sensor connected or disconnected, you can watch is moving at idle, when you blip the throttle it retards timing, then advances, then retards etc.

ECM tune is PO installed Hypertech chip, not the ThermoMaster, but the other one. It was running like a champ until the new year. Something I noticed was a bouncy tach, I hear tell wacky tachs can affect HEI, so I'll bypass the filter to see if the tach steadies, and if it does I'll get a new filter, if not I'll get a new filter and a new board. Also going to feed a 5v signal to the ESC input on the ECM to see if that corrects the wacky timing. If it does, I'll need a new ESC module, if not, then the ECM is the culprit. Personally I'm hoping its the bouncy tach, that's the cheapest fix.

EBL Locker system is coming up. I want the ability to tune the motor beyond what the stock ECM can do. The cam I chose for this build is really mild, Voodoo 256/262 with 1.6 roller rockers, I expected some learning to have to happen, but the ECM should have been able to handle that cam. It has 112* separation, so it should play nice with the stock tune and has been until recently.

The good news is I have time to figure all this out and fix the issues. I was under the gun because I didn't have a backup vehicle, now I do so I can relax a bit. The vette wasn't supposed to be a daily, but my daughter turned 16 last year and I've been letting her use the other car. I just got a great deal on a pickup truck so now I have time to figure out the issues without a time constraint or transportation logistic issues. Keep the ideas coming! And thanks for the compliment!

 

I have an 84 and found crossfireinjection.com to be a good source of information. http://corvetteactioncenter.com/foru...njection-mine/

 

I use that site as well.

 

I have used the cac site but they don't have a lot of traffic.if you have a offer of trying another ecm that's what I would do.i also would be looking at a o2 sensor, check the connection first.

 

What are u using for a real time scanner? U need something for watching real time ign advance and fuel trims. U would what this even for the EBL Locker system tuning.

BTW Howell Engine developments has some good pricing on TBI ECM systems - u may want to contact them with your problem also: http://howellefi.com/gm-tbi-products...cessories.html.

 

I went to the howel site.the fuel pressure test gage looked interesting .

 

I use WinALDL to datalog the motor and ECM. Unfortunately I can't see timing events very well, although knock counts and O2 voltages are very visible. I'm with everyone else thinking O2 sensor. Will that affect the timing though? I know it will affect fuel. My current O2 is a GM three wire (due to the headers) and as stated I'm not seeing any codes. Cardo go look at the EBL site (dynamicefi). The dataloging software that comes with that system is pretty good, monitors lots of parameters and the speed of the ECM is much better than the stock ECM i'm running.

 

I did look at the EBL w/s briefly and liked their pricing for a complete ECM upgrade. Its just that im sure most TBI owners here would like to use what they got for a moderate performance effort. And maybe u just need to replace a failed part rather than and entire upgrade.

As far as O2 affecting the timing i dont see how that happens either. Timing is more affected by the MAP, rpm and the knock sensors.
BTW Howell engineering has a TBI trouble shooting guide for only $12 - u may be interested: http://howellefi.com/tbi-service-manual.html

Good luck dude.

 

I just replaced the MAP sensor so I hope that isn't the issue. I do have a bouncy tach needle and I've heard that squirrly tachs can affect HEI distributors, so tonite I'll bypass the filter to see if the tach straightens out. If it does I'll get a new filter, if it doesn't I'll get a new board and a new filter. Picking up a new knock sensor and O2 sensor would probably be a good idea as well, they are both about 3 years old so they "should" be good, but who knows? I pulled the ESC the other day and that board was clean as a whistle. No corrosion, the card edge wasn't worn or missing any contacts and there were no burnt marks on the board or any components. So no visible damage but one of the components could be burnt out internally. The 82 part is discontinued but the 84 is still made and matches the knock sensor I have. I'll do some testing first to see if either part is bad as well as a test of the ECM. I'll put in a PM to the member that offered his spare ECM as well and swap them out to see if there is any improvement.

 

If u are buying all those parts i would recommend that Howell troubleshooting guide also. Only 12 bucks and u need some logic here so u can separate things out. Like even if u have replaced a part doesnt mean its installed and working correctly. The early TBI systems are the simplest and should be easiest to troubleshoot - usually.

Good luck.

 

I hear ya on the "usually" lol. I do need to pick up that book. I'll add it to the list. To be clear, I won't buy a single part until testing takes place to prove that the part I have is bad. As for the EBL, I've been wanting one for some time now, to give me some tuneablity for my freshly rebuilt engine. If this ECM turns out to be bad after testing, I'll just get it sooner than I thought I would. I got the new distributor because of the deteriorating wiring inside the original. I got the new plug wires because the ones on the car ohmed out very high. The MAP sensor failed the testing, the new one passed. The testing method I used for the MAP was to unplug it while the engine was running. If idle increases, MAP is good, if not, time for a new MAP. When I unplugged mine, the engine died. Plugged it back in, and tried again. Second time, the idle didn't move. When I installed the new one, I tested it the same way. When I unplugged the new MAP, the idle increased as it was supposed to. How does one test an O2 sensor? I'd really rather not replace a part that works as designed.

 

Well ya know the O2 sensor is a normal wear part. A rule of thumb (that no one realizes) is to replace the O2 when ever u change the tires/brakes. Well for testing the O2 has to be heated to NOT. Then u can measure the signal which on a OEM narrow band will swing from near zero to 900mV but the average will be 450mV for 14.7 stoichiometric a/f. On your car u can measure this at ECM black connector pin #8 to ground. My Chiltons has a circuit test for the O2 circuit with the sensor disconnected. U have to ground the ALCL connector pin B for this (diagnostic terminal grounded).
Check Engine light went off for at least 15 sec = faulty O2 sensor or connector.
Check Engine light flashing = Open loop, open O2 wire circuit, faulty ECM or ECM connector.

The Chiltons "Fuel Injection & Feedback Carburetors 1978-85" has a diagnostic flow chart for each circuit. Too much for me to type. Amazon still has them in paperback: Shipping will cost more than the book.

Hope this helps, good night.

 

I have a similar timing issue on my 82. I was going to replace the ESC with the 84 part but it looks like the connection to the ESC from the ECM is different for 84. Not sure just from pic on Rockauto.

 

Also, I ordered the knock sensor for the 84 and read somewhere that it was not compatible with the 82 ESC so I returned it.