Sanctuary

Hey guys. Been a long time since I've been here (one of the founding members), but I'm back after a bit of help from all the knowledgeable people on this great forum.

I have a 1987 C4 A4 Z52 Coupe that's been exhibiting some strange issues. Initially it was running rough and I thought that this was due to old spark plug wires (one had actually burnt through on the exhaust). Changed those, checked the timing (8 deg BTDC initial) and it did help for a while. Unfortunately within a few days it got worse again and it's now at the point where it won't fire at all. Leads are still fine.

The engine turns over with no problems (plenty of battery), I've checked there's fuel with a fuel pressure gauge (holds 40+psi no problem), air filter is fine (K&N). Next thing I did was swap out the iginition module, coil and distributor cap. No change at all. I've also tried firing her without the MAF connected just in case that was sending false signals. Also tried firing it up with the throttle wide open in case it was flooded. Ambient temperature here is good so it's not because the engine is cold.

Kinda stumped on where to go next. I do have a spare ECM, so I will try that next. Plugs could be changed but they're only 6 months old. Any other suggestions?

I have to say having a C4 down here in NZ is sometimes a pain because the local service guys look at the car and just throw their hands the the air - too complex for them I guess. So I'm just after some suggestions if you've got any before I look at bringing in outside help (which is bound to cost a lot and not actually fix the problem). Thanks in advance!

Muffin

Welcome back. Don't stay away so long this time. Have you tried the simple things? Checking to insure plugs are actually firing. You know the trick, remove plug hold it on a good ground and turn the engine over check for spark. Have you tried to start using a starting fluid? If it fires in this mode and since you have pressure at the fuel rail would indicate an injector driver problem. Do you have a set of noid lights to check injector function? These are very simple setps and checks but will eliminate some possibilities.

rodj

Back to basics
Air / Fuel /spark
One out of center that has caught people out is a bad TPS.
If reading over a certain amount (25%?) while cranking will cut injector pulse.
Also the VATS module does starter and injectors.The injector control side could be faulty.Spray some starting fluid in it and let us know the results
If enough injectors are bad they can ground the driver circuit so no injector pulse.

rodj

Clean out your message box so I can PM you

Sanctuary

PMs cleared. :o Thanks for the suggestions guys. I'll check the TPS voltage, spark and try some starter fluid tomorrow and let you know the results.

Also regarding VATS (it's by-passed on my vette), I thought that if VATS was kicking in the starter wouldn't engage anyway? Thanks again.

Scott-89

I'm having a similar problem with my 89. To date:
New Fuel Pump
New Fuel Injectors
New Fuel Filter
No codes stored
40lbs of fuel pressure
Injector test light blinks when cranking the engine over.
It will fire but won't run just eats the starting fluid.
Tried Without Maf
Tried Without TPS
Plugs and wires Distributor cap and rotor cap all less than 1000 miles old.
Not sure whats left.

jfb

Your injectors are clearly not working!

Scott-89

I was hoping that wasn't the problem as I just got them from FIC. I am not sure if there could be another cause.

I had the same problem with the old injectors and they were 20 year old multecs. It just stopped starting.

VetteFreak91

almost too easy

Scott-89

I really hope it's not the injectors. What a PITA they were to change. I would hate to think they were bad. I ohm tested 2 of them and they were both 17 ohms. I didn't test them all though.

jfb

Starter spray is the only fuel getting to the cylinders, so we know you have air, spark, and compression (the engine runs on starter spray). I believe the injectors are actuating, FIC exercises them with gas and watches them spray, but you must not have any fuel flow to the injectors, or at least one injector on each bank has a turn to turn short in their coils. An ohmeter cannot find a turn to turn short and this failure will prevent the coil from lifting the pintle on the short duration pulses the coil gets. One shorted injector on each bank will prevent each bank from working. An impedance bridge that measures the inductance and coil Q can easily find a defective coil. I found a bad injector on my 87, it measured about 1/2 the inductance and 1/2 the Q of the other injectors but the resistance was the same as the others.
I would look into seeing if you can prove that you have spray from the injectors and that the spray is gasoline and not water.

Scott-89

Well, my problem turned out to be bad gas the whole time.

Thanks for the help.

jfb

Perfectly logical. Runs on starter fluid, won't run on [crappy] gas. You had spark, air, compression, fuel pressure, known good injectors.

Sanctuary

Ok guys, I followed all the above and determined the injectors were pulsing with a light. Injector resistance is around 17.3 ohms on each. The cold start injector is around 6 ohms which seems low. Also found the original issue was no spark - replaced the cap in the distributor and now she fires. Checked TPS - right around 0.55 volts.

The problem I'm having now is the engine starts and runs for a few seconds and then dies. I've checked the fuel pressure (just over 40psi and it holds). No trouble codes are stored - I'm using Diacom and it gives me about 3 frames of the engine running before it dies and it does look like it's trying to run (advances the timing, IAC moves etc.) properly. I've also tried running the motor without the MAF connected with exactly the same results.

So my initial thoughts are bad fuel. But just wondering if anyone has any other suggestions?

jfb

Contaminated gas could still be in the fuel system, or it could have clogged your injector/s, fuel pressure regulator, or its check valve. Watch the fuel pressure gauge while it starts, runs, and then quits. If it holds up and your timing light keeps pulsing all the way till engine stoppage then you still have fuel problems but they are in the engine compartment.

Sanctuary

Yeah, the fuel pressure holds and the timing light keeps pulsing until the engine quits. I just had one other thought, what about the oil pressure switch. Is that another possibility?

Im loathing the idea of having to drain the entire tank of fuel (it's completely full at the moment) but it is over 6 months old. I know gas has a shelf life so it could be that.

Thanks for all your help thus far, it has been invalulable!

rodj

In Diacom check the fuel pump volts .That will tell if the pressure switch is maintaining power to the relay.

Run 12V to the G terminal on the ALDL ;
that should make the pump run full time, bypassing the relay

Sanctuary

Ok, thanks for that. I turned her over and watched diacom. The fuel pump voltage remains on a few seconds after the engine has stopped firing so it looks like power is not being cut to the pump. The voltage is also normal (close to the engine battery voltage read-out). You can also hear the fuel pump disengage at this time so I trust diacom.

Bad fuel? 120 litres of fuel - yey!

jfb

Well, you can keep asking questions and testing fuel pressure and spark over and over and over and ignore the logical conclusion that you have contaminated gasoline. You can save your money by not draining the fuel, but your engine still will not start and run...............but you will still have your contaminated gasoline and your money, you just won't have a car that is useable.