Shinobi'sZ

Wife went to start the 91' Z28 I just got a few months ago comes back in the house a few minutes later and tells me it won't start. After owning three of these cars I figure well it's either the fuel pump or fuel injectors. I noticed when I picked the car up it still had the stock Multec injectors and I knew it would only be a matter of time. Stuck the key in and it cranked over all of the gauges worked while cranking (oil pressure etc. could hear the fuel pump priming strong between cranks). Then popped the hood.

The normal way to trouble shoot the injectors and harness is to use a Noid Light or Multimeter to test the for 14.4 ohms. However there is another trick that can get be done quicker and I haven't seen it shared a lot so I'm going to share it because it worked for me again today. A long time ago a mechanic taught me a quick trick to get me home if the car ever failed to start or died while driving. He told me that a bad or sticking injector would short the ECM and cause the car not to start or die. I first experienced this in the early 90's with my IROC. After it died on the freeway I had the car towed to a shop to be diagnosed. After about an hour later I got a call to come to the shop when I could to talk to the service manager. Very cool shop and the service manager took me back into the service area where the car was. The mechanic was very cool and he told me that I had a bad injector and showed me how he came to the determination so quickly. While trouble shooting he disconnected each injector one at a time and cranked the engine over, when he finally disconnected the injector that was bad the engine started...albeit running on only 7 cylinders. He then turned the car off and reconnected the injector, then restarted and the car ran just fine. He told me they could order new injectors but it would take a few days and I told him I would drive the car back home and replace them myself when I got time. He told me since the injector was bad or intermittently failing injector that it could happen on my drive home. So I took the car and headed home after about 20 miles the car died and I had to pull over to the shoulder..kind of sketch now when I think about it could of happened in traffic while I was in the middle lane or something...or no shoulder..with no warning. I tried cranking a few times and the car wouldn't start...just like it did when it broke down on my while driving the first time. I disconnected the same injector that he showed me and the car started back up and I drove the rest of the way home on 7 cylinders. I did this at least one more time just to confirm the injector was bad and that disconnecting the injector resolved the issue. I ended up replacing all of the injectors with new ones and the issue went away...at least until the OEM fuel pump went out later...but that was easy to diagnose because it wouldn't even prime when I cycled the key on and cranked each time.

Anyway this morning after my wife told me the car wouldn't start and I verified i could hear the fuel pump priming between cranks. I used the same process to disconnect the fuel injectors and turn over one at a time, when I disconnected #6 the car started right up..could hardly tell it was missing at idle with the injector disconnected. I then turned the car off and reconnected the injector connector and the car started and runs fine....but if history repeats it self it will ultimately fail again. I have one CAVEAT...on my daughter 91 V6, I replaced all of the fuel injectors and fuel pump chasing both an intermittent no start and dieing issue. Nothing I tried worked, I finally read on this board of another member stating it was a bad ECM. In the end it was the ECM. When I went to replace the the ECM I noticed it had popped out of plastic holding clips behind the dash on the passenger side and was dangling by the connectors (I couldn't see it)....resulted in damaging the pin connector...replaced the ECM and it ran fine.

Back to the 5.7 from this morning's no start. I plan on modding this engine in the future with performance upgrades such as heads, cam, headers, and intake or possibly forced induction of some sort. I don't want to do it now but figure if I have to buy new injectors now I would pick up the size that I need for my future performance mods. Need to know what size injectors will support 400-450hp? If I stick 36lb injectors in my stock TPI motor for the time being is it going to run poorly or so rich that it fouls plugs? Can I replace the OEM injectors (22lbs) with aftermarket 36lb injectors on my stock motor and it will be ok for awhile until I add mods? Probably asking for too much lol.

ULTM8Z

Anything bigger than stock will require at least changing the injector constant in the chip. The closed loop fueling (where the ECM reacts to the O2 sensor) can compensate a little bit. But so much of the normal operation fueling is not closed loop, so overall itll run richer and probably give driveability problems.

And then there's the issue with Bosch-IIIs that often require further changes to the calibration due to the nature of how they operate vs the Multecs. Bosch-III types are typically what people buy these days. Its not a show stopper in that the chip changes are minimal and several members here do custom chips.

That said, if you stick 36 lb injectors in there with out changing the calibration, I guarantee itll run like carp.

Orr89RocZ

iTrader: (20)

Definitely need a chip change to account for the fuel. It will work i have run 36’s on a stock 305 cuz i had them for $100 lol. But needed some fuel tuning to be perfect.

southbay08

you will have to burn a chip to allow the computer to accomodate for the increase in fuel. Otherwise it's going to run rich and like crap

sofakingdom

iTrader: (1)

Zackly…

Some people have had success running the Frod orange-top (5.0) ones, by adjusting the fuel pressure. I doubt that this would really work "well", but might be a tolerable stopgap while sending the OE ones out for cleaning or the like. Most of the typical cleaning vendors, including the one that's a sponsor of this forum, will check them electrically, and replace bad ones. You can call and ask beforehand. I don't think those are particularly rare or anything; shouldn't be a problem for them to do that.

southbay08

we have just about every injector in stock...give us a call and we'll be more than happy to help you. We are up and running however closed for walk ins

Shinobi'sZ

So for the sake of being thorough I warmed the car up and checked the ohms on all of the injectors and put a stethoscope to each injector to listen for it pulsing. Keeping in mind that the no start issue I had was after the car sat for a week and was cold. The ohm check of the injectors read 16.2-16.4 so for the most part perfect, and all them ticked/pulsed the same when listening too. I also confirmed the ground strap on the passenger firewall to the back of the head was tight and secure.

The questions I have now are there other causes besides an intermittent bad ECM having a ground issue or something related to the injector driver that would cause a no start condition related to firing the injectors? As I mentioned with the first post, I pulled each connector off and turned over and it fired up when pulling #6 connector off....#6 ohm tested 16.4 lol. I have been driving the car most of the week short trips and a 100 mile trip. No problems....car fires right up, revs to 6K rpms when I take off going through 1-2 gears, underload lugging up a hill a little while in OD didn't miss a beat, downshifted into 3rd fine.

If an injector gets stuck does it short out the computer and cause a no start like it would with a bad injector (shorter). I'm chocking this up to bad trons as we used to call them in military aviation.

Tuned Performance

iTrader: (92)

It’s odd you don’t have a injector or two with low resistance.
If they are stock grey body multec injectors they are known to short out.
when this happens it will shut down the injector driver and you loose injector pulse to all injectors.
unless you take out the shorted injector out of the circuit by unplugging it.

sofakingdom

iTrader: (1)

"Getting stuck" doesn't really affect the resistance. Resistance (ohms) is a property of the copper that the coil inside the injector is wound out of. The coil does sometimes short; it consists of very fine wire covered with a varnish insulation, and vibration, heat, etc. cause places where the wire touches itself to wear through.

The power to the injectors is present at all times that the ignition is on. One wire at every injector is always hot. The other wire is grounded by the ECM to open the injector "valve". The ECM pulses them a few times a second for some fraction of the time, that fraction being determined by how rich or lean it thinks it needs to be. Might be anywhere from 5% of the time to 80% or more. TPI is "batch fire"; all 4 injectors on each bank are wired together and connected to one driver in the ECM. The driver has overload protection (current limiting) to keep from getting burned up by shorts; so if one injector shorts, the ECM's limited amount of current flows only through the short, and none goes through the remaining 3 injectors. That's why one shorted one takes out a whole bank and essentially shuts the vehicle down. Then when the shorted one is unplugged, the other 3 can work again, and the car at least runs, although it'll be like having one dead spark plug almost.

Shinobi'sZ

Good explanation especially for others who may stumble upon this thread with a no start issue. I also agree with everything you stated as it's been my experience with 4 Thirdgens now that when one injector goes the car just wont start because it shorts the ECM. This is why after hearing the fuel pump prime I just started disconnecting the connectors to each injector until the car started.

The perplexing part about the no start problem I had a few days ago is that the car was sitting for 3-4 days and was cold, and all of the injectors as of today read 16+ ohms. This will have to become something more than an intermittent problem and a hard failure to diagnose properly. As of right now the car is running great.

sofakingdom

iTrader: (1)

Yeah, intermittents are the most fun of all. Especially when they're only intermittent sometimes.

OTOH, if it flaked once, it's CERTAIN it'll flake again; and it'll save it for the WORST POSSIBLE instant. Like when you're in one of those neighborhoods where you don't roll down the windows, or trying to beat a train bearing down on you at a crossing. Best to give it a good healthy pre-emptive first strike.

Shinobi'sZ

Are these Delphi injectors?

Tuned Performance

iTrader: (92)

Kinda look like it, any part number on the top body of the injector ?

Shinobi'sZ

I will have to pull the connector off and check. Are they prone to failure like the Multec? Not sure if these came stock on 91' but if so they are almost 30 years old and probably have had it. The car sat for a long time I think before I got it.

sofakingdom

iTrader: (1)

Multec = Delphi = Delco = Rochester = GM

All the same. Different names for the same product, company, division, group of companies, etc.; over a period of years.

Shinobi'sZ

Thanks appreciate the time you took to answer my questions. I will probably just buy some Bosch 22lb for $140 and install them. That way I can install them the same day I pull the TPI and fuel rail off to get to the injectors. I hate having my cars disassembled waiting for parts.

sofakingdom

iTrader: (1)

Probably a good plan. Be aware though, the Bosch aren't the same height; you have to either grind down the fuel rail mount places or add spacers, can't recall which. Not a big deal but be ready.

southbay08

they look like the Rochester/multecs.
Sofakingdom...no need to grind down the fuel rail mount or add any type of spacers. The bosch lll injectors are shorter than stock however that's why we designed a custom adapter and oring that fits on the bottom of the injector which makes it a direct fit on stock set ups. We've been using them for many years.

GeneralDisorder

iTrader: (1)

The Bosch III's will need tuning for best operation. They have completely different voltage compensation and low pulse width specifications. If you want direct replacement then get Delphi.

GD

sofakingdom

iTrader: (1)

Precisely: which is why I made the comment above about that only being a stopgap.

With proper tuning however, Bosch injectors are well and widely known to be an improvement in "running" over the Multecs. The somewhat less common light blue Frod 24 lb/hr ones are another good option for the 350, since they can accommodate higher fueling requirements, if the engine is ever modded. Which is not to say that they're "better" or the Multecs are "crap" or anything of the kind (other than the Multec tendency to short out more so than most other brands); only, that the "priorities", if you will, of the 2 designs, are different. Bosch uses a pintle design that is light and fast, but relies on little holes to regulate flow which are more easily clogged and also their closing function is easily interrupted by contamination leading to "leaky" injectors. They're somewhat more maintenance intensive in that way. The Multec uses a heavy round pintle that is relatively resistant to fouling, but has other issues like slow response to electrical impulse. Like so many things in life, there's a tradeoff involved: you give up one advantage in order to secure some other one.

That said, for most of us, the Bosch are a "better" choice, because their design priorities align more closely with ours; but WILL require tuning to make really RIGHT. Not a trivial matter these days, since these cars' stock ECU can only be tuned with techniques (burning PROMs) that are a relic of the 70s.

Shinobi'sZ

It's been a while (2009) but I think I used Bosch 22lb in my 91' Firebird Formula 350 to replace the OEM injectors along with an off road pipe and it ran great. Never tuned it.

sofakingdom

iTrader: (1)

Sometimes you get lucky, but sometimes not... being the person that I am, which the only kind of "luck" I ever seem to have is bad, I've learned not to count on a repeat of any good luck that might accidentally come my way. But maybe you're different from me in that regard.

GeneralDisorder

iTrader: (1)

"ran great" is, unfortunately, a subjective measure of performance. Especially if the original injectors were spanked - in which case I'm sure it appeared to run a LOT better. But that doesn't mean significant improvements couldn't be seen from proper tuning. Also a statistical sampling of 1 is pretty weak evidence. What were the AFR's at WOT and what were the idle and cruise BLM and INT values (fuel trims)?

After setting up half a dozen cars with Bosch III's and seeing the same issues with fuel trims, etc - as well as data logging, and scope tracing the Bosch III 24's through all operating conditions and from 9v all the way up to 15v to determine latency values I can say that they will NOT run like stock injectors without tuning. Better than your set of completely hosed Multecs..... sure.

GD

Shinobi'sZ

I understand and appreciate the info and insight. My only experience really in owning four Thirdgens is there isn't a whole lot of performance to be gained with injector swaps alone. I purchased the 22lb bosch replacements, so hopefully the fuel trims will be close enough. Run great being subjective to me on the otherwise stock Thirdgens has been after replacing:
1) Does it start
2) Does it idle
3) Does it accelerate
4) Did the fuel economy get better or worse
5) How do the plugs look

Southbay told me if they don't work to my satisfaction to send them back and they will replace them with the others. They have been selling them for 13 years and haven't got too many complaints.

Now if my 1-5 above doesn't work I will be sending the injectors back. If I was to stuff a 383cid into my Thirdgen or add H&C, Intake, and CAI to it I would certainly be getting a custom tune and would be using 24lb injectors. But unfortunately I have switched out my game plan with mod'ing my Thirdgen. So now I just need to get it back to running good and it will have C6 wheels and I guess I will sell my newly purchased Dyno Don headers and just replace the muffler with a flowmaster.

Shinobi'sZ

LOL usually when something breaks on me it is always worse case scenario and ends up being the hardest thing or most time consuming to fix. I will rely on the vendor and his return policy if they don't work out. I just don't need the car die'ing on my while driving or in the middle of a supermarket parking lot with Ice Cream in the trunk.

Tuned Performance

iTrader: (92)

24#hr are to small for a 383, you should monitor your fuel trims. I bet you will run lean. If your not going to tune step up and get the Delphi’s.

Drac0nic

Do they work with Cobra 39s? I've got a TPI Engine sitting here+ a set of Cobra 39s and I'd like to add.

southbay08

is the intake stock?

Shinobi'sZ

Yeah 100% stock 91' 5.7 TPI.

southbay08

ok so you can send them in and we'll install the adapters or you can put a second oring on the bottom of the injector. Put into that little groove that's above the bottom oring