1991 ZR-1 Missing on Cold Start

[eric]

I went to start my 91 ZR-1 this morning. As I was starting it my finger slipped off the ignition and the car only made about 1/2 crank. I heard a strange noise from under the hood right at that moment. I paused for a minute then attempted to start the car again. This time it fired up immediately but was missing at idle immediately. This was accompanied by a very heavy fuel smell. I turned the car off right away and attempted to start it again about 2 hours later. Same thing.

My car is 100% stock with just over 12,000 miles on the clock.

Any ideas?

Thank you all for your time.

-Eric

91 ZR-1 Black/Grey

[secondchance]

Most likely MAP sensor hose popped off.
Map is at the back of the motor at same height as the plenum under brushed aluminum cover. With a flashlight you will see a hose from bottom of the MAP sensor connecting to the back of the plenum.
I am sure someone will post a pic if you have difficulty locating it.

[eric]

Thank you. I checked the MAP sensor hose (found a diagram on an old thread which showed a great picture). The hose was off. I can reach my fingers around to put it back on but I do not get the feeling that it secures very well to the port where it is supposed to go. I put it back the best I could and attempted to start the car again. This time it backfired again and I did not hold the starter long enough for the engine to crank over. When I checked the MAP sensor hose after this attempt, it was off once again.

What is the best approach at this point?

Again, thank you for your time.

-Eric

[Z Factor]

Hello Eric and welcome to the forum.

As secondchance said, it is most likely that the MAP hose popped off. The good news is that it is an easy fix with no cost involved. Just locate the end of the hose and put it back on.

[PhillipsLT5]

Letting go of the key early may cause MAP to blow off
Try a plastic tie on hose

[Z Factor]

Originally Posted by eric

Thank you. I checked the MAP sensor hose (found a diagram on an old thread which showed a great picture). The hose was off. I can reach my fingers around to put it back on but I do not get the feeling that it secures very well to the port where it is supposed to go. I put it back the best I could and attempted to start the car again. This time it backfired again and I did not hold the starter long enough for the engine to crank over. When I checked the MAP sensor hose after this attempt, it was off once again.

What is the best approach at this point?

Again, thank you for your time.

-Eric

I see that you posted while I was composing my post. Look at the end of the hose and make sure it is not warped or split on the end. If it is then you will just have to get another one. Chances are it is fine because the pressure will blow off a newer one as well. Just try to work two fingers back there and push it back on as far as you can with a little bit of a wiggling motion. Once it is seated on apply gentile pressure from the back trying not to deform the hose. It is similar to putting on a radiator hose in that you need to work it on slowly to advance it forward as far as it can go. It should stay on provided that you do not attempt a quick key release again.
The key is to hold the ignition on longer than what you would consider normal for your other cars. Most are just a quick 1/4 second turn and the car starts right up. With our motors you must hold the ignition over until it has fully fired up without a backfire.

[eric]

Thank you. The hose is deffinitely off. I can see/feel it clearly. It appears that it inserts into into the port but not very far. I would have thought that it would secure firmly into its connecting spot, but it doesn't. I don't want to put too much pressure on it.

It feels like it would just blow off easily again.

I don't mean to sound ignorant....but am I missing something?

[tomtom72]

Eric,

The hose is designed to pop off upon a back fire.

Two things here.
The most important one is "hold the key over at the 'start' position" on a start attempt and do not let it return to the "run" position until the motor is running.

The second thing is that oil blow by from the PCV system ends up being sucked back into the plenum and a good deal of it ends up inside the MAP sensor hose. I usually use throttle body cleaner to clean the inside of the hose. You can just blow thru it to get any oil out of it also. I clean the ends with a paper towel and stick it back on.

This odd starting behavior is a result of our ignition system being the first system used by GM to not use a distributor. The ECM has to have received and verify signals from the Crank and Cam sensors before the ignition system actually does anything. Hence the "hold the key over till it fires" deal.


Tom

[secondchance]

Originally Posted by tomtom72

Eric,

The hose is designed to pop off upon a back fire.

Two things here.
The most important one is "hold the key over at the 'start' position" on a start attempt and do not let it return to the "run" position until the motor is running.

The second thing is that oil blow by from the PCV system ends up being sucked back into the plenum and a good deal of it ends up inside the MAP sensor hose. I usually use throttle body cleaner to clean the inside of the hose. You can just blow thru it to get any oil out of it also. I clean the ends with a paper towel and stick it back on.

This odd starting behavior is a result of our ignition system being the first system used by GM to not use a distributor. The ECM has to have received and verify signals from the Crank and Cam sensors before the ignition system actually does anything. Hence the "hold the key over till it fires" deal.


Tom

Exactly!
This hose has a hard plastic bent portion w/ regular soft vacuum hoses at both ends to slip over the nipples.
Over time they soften up and get oily.
You can buy a new one and degrease the nipple on the plenum, slip a new one over it and zip-tie it as an additional insurance.
Do not replace w/ regular vacuum hose!
This can be replicated quite cheap with combination of plastic tubing and vacuum hose sold at any autopart store (Advaced Auto Part, Discount Auto...).

[eric]

Thank you everyone for all of the advice. I will try to secure the hose tonight when I get home.

This car is a recent purchase for me and I am looking for any reccomendations for shops in the Scottsdale/Phoenix area who are capable of working on the car should I need assistance down the road.

Thanks again for the help!

-Eric