Ranbo108

Greetings to All,

I have a problem with a '91 Z28 305 TPI setup that has me really stumped. Basically stock engine with a cam and a custom chip from a well known local guru.

Riding along one day, the engine simply quit and would not restart. Towed home and next morning it fired right up. Hesitant to trust driving the car, I did a short ride around the neighborhood and again it quit after about 5-10 minutes. Next day, started up and brought it home.

As of now, it will start and run for about 2 minutes and quit. The injectors stop firing even though there is plenty of fuel pressure. Spark is good, it just isn't getting a ground pulse at the injectors. 12 volt supply is consistent at the injectors, just not getting the ground pulse.

So far I have replaced the distributor and coil (needed this anyway) and used TWO known good ECMs with the same custom chip and have the same results. I have scoured through the wiring at engine and ECM and cannot find any connection problems.

If anyone has a suggestion or has ever dealt with this condition, please chime in here.

Thanks, Randy

roysatikas

iTrader: (1)

Just a thought as I'm not a TPI guy. I had the same issue with my AC and finally tracked it down to the AC relay, ac clutch activated fine switching AC on / off at control panel until engine was completely warmed up, shut down the engine, start it again and switching AC on would not engage clutch. Relay got warm and would not work, is there a relay that controls ignition or anything that may cause this becoming faulty when hot?
Good luck, our relays are 23 years old if you have the last 3rd gen!
Roy

RBob

iTrader: (1)

If the stock OEM Multec injectors are still in the engine, replace them.

RBob.

SbFormula

The injectors stop firing even though there is plenty of fuel pressure.


How do you know they stopped? Just curious.

Ranbo108

I admit, I'm assuming the injectors aren't firing since squirting fuel into the intake, it will run and there is a constant 12 volts to the injectors. I simply don't know how to validate this condition. Thanks

RBob

iTrader: (1)

Can use a NIOD light, or measure the resistance of the injectors. Any under 12 ohms is bad. This is how Multecs fail, the coils short out and the ECM can't fire the injectors. There is only one injector driver for all 8 in the '86 - '92 TPI ECMs.

Hence my previous post of "If the stock OEM Multec injectors are still in the engine, replace them."

RBob.

Ranbo108

RBob, I think you have cracked the code for me! This makes absolute sense, now. When I assembled the TPI engine parts for this car, I did use a "good" set of Multecs which I had flowed and checked ohms. That was around 5,000 miles ago and almost 8 years. It is highly likely that at least one of these bad boys have hit the skids. Since I have been forced to store the car outdoors since 2010, I can imagine weather has taken it's toll as well. You can be sure that the first thing I do when I get off work is ohm those injectors.

You have filled in the blank in my understanding of the system; Thank You

SbFormula

Awesome! Make sure you test them when hot as well. I would ohm them cold and they would read good then activate them with direct 12volts DC (rapid on-off pattern) and the ohms would go down, indicating bad. Some would go down as much as 6 ohms.

Ranbo108

Got it, thanks for the tip!
I'll follow up with the results

TTOP350

iTrader: (8)

You sure there isn't two inj drivers? My service manual shows 2 and has 2 fuses, one for each inj bank and one for each driver.

RBob

iTrader: (1)

I'm sure, one driver. It does exit on two pins from the ECM, but both pins are connected to the ONE DRIVER.

RBob.

Ranbo108

Well Fellas, all I can say is Thank You, once again, for sparking my mind onto the right track. Sure enough, one bank had one injector with 4 ohms and the other bank had one with 0 ohms. I left those two disconnected and it fired up and ran... like a 6 cyl. Ha Ha! I can hardly believe that I overlooked this simple test yet I suppose my focus was elsewhere.

I bought the PROM and new wiring harness from Jim's Performance in Ellicott City, MD. Jim is a great gut and once worked for GM. He tested the injectors for me back in '06 and I simply didn't suspect they were bad.... that is until I read about the effects of ethanol fuel specifically on the Multec design.

I ordered a set of Bosch type II to replace them and look forward to having this machine whole again.

RBob, if you're ever near 21061, look me up and lunch is on me. Thanks again to all who replied. This has to be one of the best sites around for helpful members.

Cheers, Randy

TTOP350

iTrader: (8)

Then Y would it need 2 fuses and when 1 blows it takes out one bank?
Is the driver internally seperated into 2 ??
(just curious to know, always thought there were 2)

SbFormula

If 1 fuse blows it takes out one bank because it shuts the current. It does not mean the driver stops grounding.

bigal55

iTrader: (1)

From an electrical standpoint, eight injectors firing all at once requires a good amount of current. Automotive wiring can't be big and stiff/bulky which is what's required to carry high currents safely. Two smaller wires and the fuses to protect them are easier to route and connect among other advantages. Just my