natgariepy03

I've recently swapped a 1987 305 tpi engine into my 77 corvette and I am having troubles with the timing being way too far advanced. I set the base timing to 6~7 degrees and the car runs fine when not moving, but when I plug the brown and black wire back in (I cant remember the name of it right now) the timing advance is maybe 30 degrees. This causes the engine to stall more often than not when I put it into gear, and the car is currently undriveable due to that. I've narrowed the problem down to either the electronic spark control module, of which I have ordered a new one and will install it as soon as I get it, or the lack of a VSS. Does anyone know if a missing signal from the VSS would cause this huge advance in timing? I know not having one may cause trouble while the car is moving or coming to a stop, but I've read conflicting information on if it's needed for the car to idle properly.

Also, does anyone have any links to a 2000 ppm vss that fits into a th350 trans?

Thanks in advance for advice and suggestions.

RBob

iTrader: (1)

Any SES/CEL codes? Should have 42 from opening the EST/BYPASS and running the engine.

Are you sure the timing is advancing and not retarding? Advanced timing rarely causes stalling, while retarded will.

The electronic spark control module (ESC) is for knock detection and reporting. It can't cause the ECM to advance the SA, only retard it.

As for a VSS, best bet is to get a 2K PPM digital inline unit.

RBob.

Tuned Performance

iTrader: (92)

https://www.ebay.com/itm/36392862266...mis&media=COPY

natgariepy03

RBob,
As for the codes, I don't have a way of getting them because I do not have the entire electrical system from the donor car. (it was totaled in the 90s and my uncle pulled the engine and computer, which sat in his garage until I picked it up a few months ago)

I'm fairly confident the timing is in fact advancing, as the mark on the harmonic balancer is pointing about straight up when the timing light fires. If it was retarding it would have to be retarding over 270 degrees which is beyond the limits of the electronic adjustment according to my research. I have not however watched what happens to the timing as it stalls, but the engine behaved the same way with and without the knock sensor plugged in.

I installed the new ESC today and it looked promising for a while, as the timing advance was about 12 degrees when the engine was idling cold after I first started it, but after about a minute of driving it started stalling again and when I returned to the garage after driving long enough to warm the engine up the timing was way advanced at idle again.

For a little more context, I have a new o2 sensor and MAF because those were missing when I got the engine, I just installed a new coolant temp sensor, idle air controller, and now the ESC, and I checked the TPS voltage (it was about .5V closed and 5V wide open). My friend and I have made a complete custom wiring harness that uses the original plugs and most of the original sensors, but we have checked our work front to back several times, even finding mistakes in the wiring diagrams that can be found online, and we are positive that everything is wired correctly.

As I am writing this, I just realized that the MAF could have been damaged from multiple backfires out the intake. I'm not sure if the sensor is that sensitive and fragile, but if I can't find a solution in the next couple days I may just order another one.

Tuned Performance,
Thanks for the link to the VSS, it is much appreciated.

natgariepy03

Update: Issue possibly found!

Today while messing with the car after work I messed with the MAF, which seems to be fine, and then decided to unplug the cold-start injector. This seems to have drastically lessened the issue, and the car was driving fine with no stalling issues for a while until it warmed up. This leads me to believe that the injectors are sticking open and dumping fuel into the engine, which may explain the timing being so far advanced, as well as the stalling under load. I have put injector cleaner into the gas tank and I will also get my hands on a new set of injectors. Hopefully this is actually the issue and I'm not just throwing more parts at it that it doesn't actually need.

natgariepy03

Another update: issue not resolved.

I've installed the new injectors, and the problem still exists when the engine gets to operating temp. Upon first startup when the engine is cold, the timing is advanced to 12 degrees, 6 more than base. Once the engine warms up however, it idles with an advance of about 30 degrees and stalls consistently. I'm starting to wonder if the computer is bad.

Tuned Performance

iTrader: (92)

Est disconnected or connected when your seeing the advanced timing ?

natgariepy03

Tuned performance,
Connected, when disconnected the timing reads a steady 6 degrees.

Tuned Performance

iTrader: (92)

What’s main sa lv8 table calling for ?

Tuned Performance

iTrader: (92)

This is abun for example

natgariepy03

I haven't read anything about that table, after a quick look it seems like it's related to spark latency? I just set the base timing to 6 as that's the number I found when researching stock specifications.

Tuned Performance

iTrader: (92)

I don’t understand the issue?

natgariepy03

Yeah I think I've been a little confused. I guess it could be advancing correctly according to your table (I must have found something different when I looked it up). I didn't realize that it would put so much advance in everywhere throughout the curve, and I hadn't read anywhere that it should be doing that. If that much advance is expected I guess the only problem is stalling when the engine gets hot, and if it isn't timing or fuel related I am not sure what it could be.

Tuned Performance

iTrader: (92)

Heat related stalling could be a vapor lock, icm or injectors shorting and turning off injector driver. Just a few possibilities.

Komet

What is your fuel pressure when the engine stalls out?

natgariepy03

Tuned Performance,
Thanks for the suggestions, I will check on those

Komet,
Not exactly sure, I've got a 60 psi in-line pump and I'm running the stock pressure regulating diaphragm. Is it possible to monitor the pressure in the rails from the little valve with a tire pressure gauge? If not I will have to install a gauge on it.

natgariepy03

I got a pressure gauge and drove around with it, it is at 45 when the engine is off, 37-38 when running under low load, and 45 when under load.

Here's a link to a video demonstrating the engine stalling. This was taken after driving for about 15 minutes, the car is in park and I am not touching the throttle.

Komet

Fuel pressure looks good, but I noticed your engine is stumbling around long before it quits; that's a problem. They do idle a little high like that after restarting, but eventually it should go down to like 650rpm and stay there. Have you checked it for vacuum leaks?

natgariepy03

I haven't checked, but I have taken the intake plenum off a few times so I'm sure that everything is hooked back up. I suppose there could be a leak somewhere in the stock vacuum system on the corvette, I'll try plugging the ports on the intake and see what happens. I also have a vacuum gauge so I'll hook that in somewhere and see if I'm getting low vacuum for any reason. I will also try spraying something flammable onto the intake gaskets, hopefully it isn't that, I don't want to take the entire intake apart again

natgariepy03

Vacuum gauge reads just over 20 in Hg both with every port plugged and with the vacuum system hooked up, and spraying starting fluid at the intake manifold gaskets did not cause any surges.

natgariepy03

It's been a while, so here's an update on what I've done on the car the past week:

I found that oil was leaking into cylinder 2 quite badly, and a couple others not so badly, so I changed the valve stem seals. Not very surprisingly, this did not fix the problem. I also hooked up an oscilloscope to the injectors and verified that the pulse is there and does not disappear when the engine stalls. I also hooked the oscilloscope up to the tps and verified that with the key on, it reads .4V closed and 4.5V WOT. I also left the oscilloscope hooked up while the engine was running and the reading was consistent. I then unplugged each of the engine sensors one at a time and ran the engine. The problem did not go away until I unplugged the tps, but I believe that's because the idle speed did not ever drop lower than 1500 rpm. Is it worth buying a new tps because that is the only sensor that hasn't been replaced, or could the problem be computer related? At this point both I and my friend who has been helping me this whole time are completely stumped.

Side note, when I did the valve stem seals, I found that not a single one of the exhaust valves had any seal besides the tiny rubber o-rings, and every intake valve had one of the rubber stationary seals. Is this stock or has someone gone into the engine before me?

Komet

Leakdown test might be a good indicator of what kind of problem you're looking at. Smoke test is usually more reliable than carb cleaner for vacuum leaks. Not being able to read the codes and get a datalog is a pretty big handicap. 0.4v on the TPS is low, but 0.5v is within spec.