88 starts up cold but not hot

just started doing this last week. i can start it right up when cold and drive straight 60 miles to work and runs like a top. but if i stop to get a cup of coffee or gas on the way and shut it off it will not start back up for about 10-15 min. when it does start back up it sounds like it's fireing on the wrong cyl a few times when cranking it over then fires up and runs like a top again. i plan on attacking it this week end, any help on where to start?? will a bad CPS do this? thanks for any input.

When it won't start, try pouring water on the crank position sensor CKP to see if cooling it off makes it work again.

Leaking injector?

MAT sensor giving bad readings when hot.

IAC sticking, or not reseting properly when hot.

B+ relay is a delay off timer that resets the IAC for the next restart after key off.

Does it ever stall while you're driving or only not start once shut off?

Here's how to test the sensors. Make sure you have a no start condition.

http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

what i did to it last weekend

new/rebuilt injectors
plugs/wires/cap/rotor
no change

the rig has 255,000 miles but rebuilt motor has 65,000 would it be wise to pull the TB off and clean everything first or try and find the problem first by testing everything. Thanks

Saudade said:

Does it ever stall while you're driving or only not start once shut off?

Here's how to test the sensors. Make sure you have a no start condition.

Click to expand...

it has shut off one time when sitting at a light but started right back up. it seems to happen when shut off after hot for like 5 mins/ grab a cup of coffee/ then you have to wait 10-15 mins to get it to start back up.

baybilly said:

it has shut off one time when sitting at a light but started right back up. it seems to happen when shut off after hot for like 5 mins/ grab a cup of coffee/ then you have to wait 10-15 mins to get it to start back up.

Click to expand...

Sounds very much like what my 88 would do. It turned out to be the CPS. The resistance would change with the temperature, I'd test it cold and the ohm values would be within spec. but when it got hot the resistance would climb to very near the allowable tolerance. I also had some dirty connector issues going on, that likely added to the total resistance between the sensor and the ECU.
On a similar note, I had one CPS that was getting covered in hot oil from my rear main seal leaking. It also acted much the same way yours seems to be acting.
Pouring water on mine to cool it down would usually get me one more start. Or like you do, waiting for it to cool down some would get me another start.

I'm with the others on the CPS/CKP, it is most likely failing after reaching high temps.

Sounds like the heat soak after shutdown allows the temp to rise just enough. The crankshaft position sensor is such a common problem, I check that first as the others have suggested.

8Mud said:

Sounds very much like what my 88 would do. It turned out to be the CPS. The resistance would change with the temperature, I'd test it cold and the ohm values would be within spec. but when it got hot the resistance would climb to very near the allowable tolerance. I also had some dirty connector issues going on, that likely added to the total resistance between the sensor and the ECU.
On a similar note, I had one CPS that was getting covered in hot oil from my rear main seal leaking. It also acted much the same way yours seems to be acting.
Pouring water on mine to cool it down would usually get me one more start. Or like you do, waiting for it to cool down some would get me another start.

Click to expand...

ok, checked the ohms cold on the CPS 204
ran for about 5 mins ohms 217
then ran it up to temp ohms 232
turned off and let it sit one min ohms climb to 244 then started to come down with cooling of motor as it sat. this right??

And the plot thickens...

Specs are 200 +- 75 so a range of 125 to 275 is good.

Scrub the CPS connector out good and if you have one, the second connector between the CPS and the ECU, clean that one also. I use a tiny scrub brush (for brushing between your teeth) and a quality contact spray. I found just spraying out the connector doesn't always get the job done.
Mine has kind of a unique setup, I have a C101 bypass kit for my ECU (export, partially bypasses some of the C 101 connections) I'm not sure if they all do or not. If your CPS wires run through the C 101 plug, I wouldn't mess with it, I'd hack off the wires and then solder them into the harness a few inches from the firewall where the ECU harness goes through.
May be the 244 ohms you have, plus a little resistance from a connector may add up to trouble.
I did a resistance test between my CPS connector and my ECU connector.
It may be something else, but I learned a long time ago resistance in the harness or connectors often causes grief. It's worth a look.
You also have to fish out all of the extra CPS wire and inspect that, I ran mine along with the speedo cable (with cable ties). Uses up some of the extra wire and helps keep it out of harms way. If you do this, leave enough CPS wire loose so when the engine moves around, it has a little slack. But not so much it can rub someplace or flop over onto the exhaust manifold.
IAC can also be temperature sensitive, they can stick hot or cold. If you give a little pedal like 1/8 th and hold it during start up and if it starts, this is almost surly a sticky IAC (or low battery voltage).

8Mud said:

Scrub the CPS connector out good and if you have one, the second connector between the CPS and the ECU, clean that one also. I use a tiny scrub brush (for brushing between your teeth) and a quality contact spray. I found just spraying out the connector doesn't always get the job done.
Mine has kind of a unique setup, I have a C101 bypass kit for my ECU (export, partially bypasses some of the C 101 connections) I'm not sure if they all do or not. If your CPS wires run through the C 101 plug, I wouldn't mess with it, I'd hack off the wires and then solder them into the harness a few inches from the firewall where the ECU harness goes through.
May be the 244 ohms you have, plus a little resistance from a connector may add up to trouble.
I did a resistance test between my CPS connector and my ECU connector.
It may be something else, but I learned a long time ago resistance in the harness or connectors often causes grief. It's worth a look.
You also have to fish out all of the extra CPS wire and inspect that, I ran mine along with the speedo cable (with cable ties). Uses up some of the extra wire and helps keep it out of harms way. If you do this, leave enough CPS wire loose so when the engine moves around, it has a little slack. But not so much it can rub someplace or flop over onto the exhaust manifold.
IAC can also be temperature sensitive, they can stick hot or cold. If you give a little pedal like 1/8 th and hold it during start up and if it starts, this is almost surly a sticky IAC (or low battery voltage).

Click to expand...

I went ahead and put in the new CPS and ran the same test as before
cold ohms 212
5 min running 220
up to runnig temp 231
not as high as the old one.
I'm going to go over all conectors now, thanks for the help, i'll be back

Check the MAT sensor hot and cold, against FSM specs, as I suggested earlier.

Also, note that the renix has a critcal minimu cranking speed, so if it cranks slower when hot, then faster after a cool down, look for why it is cranking slower when hot.

Ecomike said:

Check the MAT sensor hot and cold, against FSM specs, as I suggested earlier.

Also, note that the renix has a critcal minimu cranking speed, so if it cranks slower when hot, then faster after a cool down, look for why it is cranking slower when hot.

Click to expand...

MAT seems to be working. i'll have to recheck when its back up at running temp. as i'm cleaning the conectors the ohms ar going up like they should. i did find the screws loose on the IAC, not falling out loose but not tight. the bat is new and cranks the same hot or cold. Thanks for the help.

MAT ohms at 300 after driving it 20 miles and up to running temp, let it sit for 5 mins and it started right back uparty:the first time in two weeks, let it sit another 5 and started right back up, me so HAPPY now. i hope it is fixed. i spent all day on it with the help of you guys and a big THANKS to you all for the help. not real sure which one fixed it, maybe a combo. the list in order

tested the CPS from cold to hot/ within specs but a little high when hot
installed new CPS and tested it/within specs but lower ohms when hot.
cleaned all conectors
pulled IAC and found loose screws/ cleaned IAC and TB with TB cleaner
tested MAT when motor was cooling down and ohms went up like they should
took for test ride then shut it down and tested MAT 300ohms at running temp
let sit for 5 mins and it firered right back up like it should.

so not sure which one fixed it. i'll let you know if it stays fixed in the next few days.
BIG THANKS you guys for the great help.:clap:

I pulled my MAT and it was seriously crusty. Which almost has to have some affect on proper (and timely) temperature readings. I carefully cleaned off the sensor end with solvent.

8Mud said:

I pulled my MAT and it was seriously crusty. Which almost has to have some affect on proper (and timely) temperature readings. I carefully cleaned off the sensor end with solvent.

Click to expand...

i will do that in tha AM. Thanks for the help.
i'm going off this
http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

is this outside air temp or engine temp when doing the MAT test, makes a big difference in out come. sorry for the dumb Q but i'm new to the Renix system.

8Mud said:

I pulled my MAT and it was seriously crusty. Which almost has to have some affect on proper (and timely) temperature readings. I carefully cleaned off the sensor end with solvent.

Click to expand...

Sounds like a good routine PM practice.

baybilly said:

i will do that in tha AM. Thanks for the help.
i'm going off this
http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

is this outside air temp or engine temp when doing the MAT test, makes a big difference in out come. sorry for the dumb Q but i'm new to the Renix system.

Click to expand...

It reads the air temp of the air passing the sensor. So with engine running it will be air that is between the intake manifold temp and ambient (variable, and varies with rpms!). Engine off it should read the same as the manifold area it is threaded into.