Wont start when hot, Starts and runs cold fine

[jjsfollys]

86 Camaro 305 TPI setup, starts and runs fine when engine is cold. When it warms up even a little, it will not start. I have replaced the Ignition Module, (which was already new), with a known good one and another new one, no help. I have replaced the harness at the distributor, as when I tested the no start, I could get the injectors to pulse manually and the module has been eliminated as the fault, no help.

When it is cold all the injectors have good pulse and the engine starts and runs great, with no trouble codes present. I checked the timing and set a trouble code. I tested the TPS and disconnected the MAF when it would not start. I installed a known good ECM and Prom. I am still at square one. Nothing seems to be a problem, yet it still has no injector pulse and will not start until the engine cools to cold.

I also checked all the engine and ECM grounds, and verified all the battery voltages at the ECM. I am confident the problem is something well outside of the norm. I have seen and repaired some pretty tough problems in the past, this one is biting me. Any help is appreciated!

[Blue Bowtie]

Welcome Aboard!

Is there spark at the plugs when cranking the engine hot?

As you probably know, you'll always set a '42' when bypassing the EST to set base timing. That's not a problem. Clear the code just to avoid confusion, and in case the EST/KS have a problem.

How is the signal from the pickup coil/pickup coil resistance when cold/hot?

I've also seen this with a cracked reluctor.

[jjsfollys]

Okay, here it is... I did some more diagnostics and this is what I found. The plugs continue to spark even though there is no injector pulse. This only indicates that the secondary side of the distributor is good. It was very difficult to test the pickup because of where it is located, but it was original and was looking rather poor.

I pulled the distributor and replaced the pickup with genuine AC Delco. I reset everything and still starts and runs fine when the engine is cold and continues to run great when hot, no codes, stabile idle with great throttle response, but will not start if you turn it off and try to restart. When engine gets cold it will start again just fine.

I grounded the cold start injector and 'wallah' it now starts normal hot or cold. The cold start injector is on an entirely seperate circuit from the other injectors and is designed to 'enrich' the cylinders when the engine is cold to help it start easier in cold weather. The cold start sensor will only allow the injector to run for eight seconds and then it cuts off. It also has a temperature sensor thet cuts it off when the engine temperature is 95 or above. The engine relies on the port injectors to be able to start it under normal operating conditions. These injectors get their pulse from the ECM via an input signal from the Ignition Control Module. I Ohm'd the harness from the distributor to the ECM and although I am unaware of a spec, it all tests normal. I replaced the coil side of the harness and the radio noise suppressor previously. I tested the 8 injectors, and tested for pulse on both sides while all were disconnected to eliminate a shorted injector, (none). I even tested the O2 sensor for a short, even though it too is new Delco.

The port injectors should pulse, even at the 450-500 rpm the starter turns it over, but they do not. With the cold start injector to enrich the cylinders, it will start every time, and the port injectors pulse as soon as the engine starts. I cannot understand this. It makes no sense at all.

I refuse to believe that in 20 years of these TPI systems being around that I am the only one to have this problem. What say you?

[wrightz28]

That is bizzare, but not ultimately unheard of.

[jester151315]

not too likely, but your problem may be vapor lock. im sure you know this is caused by having a fuel that vaporizes too quickly. if this just started to happen after the winter, and you dont drive it much and still have some lower octane fuel in your tank it may be vaporizing before it reaches the injectors.

[Scrapper]

have you changed coil?? and when it gets hot did you check for spark? if you still have spark i'd say fuel pump...no such thing on vapor locking like i said it's a old myth....

[jester151315]

did you just say vapor lock is myth??

[jjsfollys]

Thank you all for your input. I have dealt with vapor lock some years ago on an old Dodge Reliant K car. The fuel line was routed too close to the strut tower and manifold causing the fuel to boil in the line. It would run great for about 30 miles then just shut off. It would not restart until the car cooled, usually about 45-90 minutes. A simple reroute of the fuel line and insulating it with some old heater hose corrected the problem. It is rare, but it does happen.

No, I am positive my problem is not vapor lock. I do not believe it is possible on a high pressure system anyway. This TPI system runs 40-47psi constantly. I have a gauge hooked up to the system and it never loses fuel, the pump does not bleed off and the pressure regulator is functioning properly.

It is not losing spark, and I have not replaced the coil. The coil is entirely secondary ignition and the problem is no injector pulse when cranking. Being port fuel, each cylinder has its own injector. The injectors are controlled on the ground side by the ECM. The ECM receives a reference signal from the module telling it when to pulse the injectors, (or provide a ground signal). There are 2 banks, essentially left and right. Each bank is powered by a seperate fuse that is hot all the time. My injectors never lose power, they just are not being grounded by the ECM, so they do not spray fuel.

The thing I am having trouble with is that when I test the entire circuit, it all functions normally. Meaning, if I manually pulse the module, the injectors pulse. This tells me the module is bad. As I already stated, I installed a new one. No pulse. I installed a known good one, (it worked on another car). No pulse. I purchased another new one from another source. No pulse.

I replaced the ECM, with another known good one. No pulse. I replaced the PROM. No pulse. I just replaced the pickup coil (or pole piece), yesterday. No pulse. If I pulse the cold start injector, the cylinders fire, the ECM pulses the injectors and it runs until I turn it off. 2 minutes or 2 hours. Once you turn the key off and try to restart it, no pulse. Add fuel through the cold start circuit (hot or cold), it starts right up and it is good again until it is turned off.

I apologize for being lengthy, but I am hoping someone has either seen this problem already or thinks differently than I and can lead me in the right direction.

[jester151315]

well if it means anything i got to a tech school and my teacher loves questions like these, so ill print it and ask if he has any thoughts

[teenasparkle]

Hey!
Sounds like you've run into the famous ECU conundrum! Welcome to the club. Yeah - the short answer is what you've already done - shove some fuel into the engine DESPITE the wonderful injector control default (NOT - you'd think they would default it to a minimum idle amount like 5% mark/space or so - but no such luck). Great things the cold start injectors! Lets you bypass all the useless electronics. Just hit the switch (create your own on the dashboard) and she'll start - cold OR HOT! Isn't it great that when you're out in the wide blue yonder these days you not only need the usual toolbox to fix your car, but also A COMPLETE ELECTRONICS LAB REPLETE WITH OSCILLOSCOPES ETC.
Best Regards and HAPPY MOTORING!

[Tech II]

This post is 8 years old.....teenasparkle, you have peaked my interest....have never heard of this ...what is causing the injectors not to fire?

[j cAT]

Originally Posted by Tech II

This post is 8 years old.....teenasparkle, you have peaked my interest....have never heard of this ...what is causing the injectors not to fire?

I just happened to see this as I was looking at the forum posts. having read this post from teenasparkle I knew it was a woman ! pure nonsense..

[teenasparkle]

Hi guys - yeah just a bit more info for them what's partial. I note that one response was, quote: "... I knew it was a woman ! pure nonsense.." - Well now that goes without sayin, don't it?

ECU Issues

No start is a good one. Most of the no start issues occur when the engine is hot. The ECU in its computer-nerd programming wisdom has decided to give the injectors NO PULSE AT ALL. So you can’t even start the car. Beautiful. Website forums are full of this issue. ECU’s are programmed for optimum fuel consumption and least pollution, but they should have an emergency bypass allowing the car to operate, albeit with a prominent dash display indicating a problem. None do. But if they did the car would work, as opposed to not work. That could be important. Any efficiency aspects, if there were any, could be fixed later.

Make sure the electrical system works first (take out a spark plug, tie a wire from the spark plug case to a metal part of the motor, crank the motor, see if there’s a spark).

Also make sure you’ve got fuel pressure (fuel pump on and working properly when cranking).

If the problem is still fuel-related (spark plug dry when removed), to fix the classic no-start when hot or even cold problem can be relatively easy, depending on your car. Find the ‘cold start injector’ if there is one (some cars no longer have them). This is not one of the normal pulsed injectors for each cylinder that run on an ECU-generated frequency. The ECU runs at about 10Hz with mark/space control to regulate the fuel. You can see that on an oscilloscope. The ‘cold start injector’ looks similar to the pulsed injectors but can be hidden somewhere under the manifold. Trace the fuel rail linking the normal injectors and see if it goes somewhere else, which is not always easy. Use a good torch. When you find the ‘cold start injector’ you’re in business. (If there isn’t one the ECU runs the cold start function in the pulsed injectors, and I’m afraid you’re stuck with the nerdy nerdies).

Take the electric plug off the ‘cold start injector.’ You may have to push a release. Try to get a similar plug to avoid cutting the wires which is a bit non-reversible without a soldering iron. If you can get another plug, simply wire one terminal to the engine or chassis and the other one to the starter activating terminal on the starter solenoid. The injector draws about 1 amp current, so the extra load on your ignition switch starter terminal should be OK. Sometimes there’s an extra lug on the solenoid that comes on only when the starter cranks (but doesn’t drive it) – if you can find one of those that would be preferable. Make sure the ‘cold start injector’ ONLY COMES ON DURING STARTING, otherwise you’re not going to be kind to the environment or your fuel bill. This is only a temporary fix – as soon as you can you’ll have to get your ECU with all its peripherals checked out by the auto clinic (bring your wallet).

Happy back on the road.