1997 trans slip and boil over on highway

140,000 miles

Car is used as daily driver 25 miles each way, rarely used for more. A couple of times last summer I had driven further(50miles) and had trans slip and the computer coded trans slip...P1870? Trans fluid drain, filter change, fill last summer. Seemed to be OK.

Last fall...Highway speed and driving for a couple of hours and all of a sudden my trans starts to slip. I pulled over and the coolant was boiling over. Fluids were OK. Let her rest for a while and made it the rest of the way home with HEATER RUNNING, no problem. No service done for problem since. Heat related...fall...no more trips...flirtin' with disaster but has been averted.

Couple days ago...was stuck in traffic for 20 mins. Out of traffic and driving another 20mins...stopped at light....boil over...engine rough but got home...5 miles. It's parked.

Service since last fall...just regular change oil.

I was thinking the two common points of last fall's problem was radiator.

Today...warmed up vehicle, circulation is OK fluid looks kinda brownish with floatys. No major gummies but a little OIL SLICK on top. Temps are even around rad(190) as read by infrared temp sensor so I'm not sure there's blockage. I did notice some cottonwood fuzz on about -25% of rad.

No intake plenum change to date.

Would trans problem cause boil over? I've read my trans problem could be TCC related. Or is the fluid not getting cooled enough causing slip? Trans fluid doesn't smell burnt. Should I just flush and fill coolant, clean the fuzz from the front, and see how it goes? Should I just get a new radiator since that's the common failing area? Looking for a little advice before spending.

Thanks for reading this and maybe helping. All advice and solutions welcome!

BTW no temp light comes on when overheating but it does come on when starting the car.

You need to diagnose the cooling system,have the rad checked for proper flow, see if Dex cool is jelling, If the trans is slipping and setting that code,this is a seperate issue that needs diagnosis also.

MY(always DIY even though I'm no mechanic, and probably clueless) diagnosis was to start the car cold, warm up and monitor the temp across the system. When the thermostat opened, there was an increase in flow with the cap off. I put the cap on read input and output hose...170 both sides. Rad read 198 for as much as I could read behind the fans that I had unbolted from the top shroud.

Does a rad shop measure flow rate and is there a way for me to do it?

Guess what I'm saying is..."in the shop for sure?"

Thanks for your reply, Maxwedge.

There should be a 20 degree minimum difference between the inlet and outlet hoses if the flow is good and the coolant is not contaminated. I don't like the sound of the oil contamination you describe, old Dex has been noted for ' jelling' up. If you flush the coolant see if it is getting mucky this indicates what may be happening in the cooling system. If never done it is way overdue!

OK. I drained and ran flush through the system. When drained I could see the sides of the rad and they looked OK. The trans coooler coil had some sludge on it but not entirely gunked over. Took out thermostat before flush and the interior of the motor looked OK. Stat checked out OK in the kitchen. Took reservoir, which is nasty looking(coated with unwashable gunk) off and noticed that the hole was restricted somewhat. Noticed constant drip out of reservoir drain on rad neck so I need a new cap. While running the flush the temps were about 15 degrees apart in/out. Temps seemed pretty even across rad. Used the backflush system to get everything out. Then the bummer...

Still have the oil slick on the otherwise clean, backflushed water. I know this isn't good. I was hoping(?!) it was a leak in the trans cooler but I took off the top hose and got the water from there and sure enough, oil slick. So......how do I know if it's head gasket or just(HA!) the intake manifold procedure. I've read pressure test. Does that determine my next move? Is that something I can do? Does anybody have a link to procedures?

Thanks for your help.

OK. I drained and ran flush through the system. When drained I could see the sides of the rad and they looked OK. The trans coooler coil had some sludge on it but not entirely gunked over. Took out thermostat before flush and the interior of the motor looked OK. Stat checked out OK in the kitchen. Took reservoir, which is nasty looking(coated with unwashable gunk) off and noticed that the hole was restricted somewhat. Noticed constant drip out of reservoir drain on rad neck so I need a new cap. While running the flush the temps were about 15 degrees apart in/out. Temps seemed pretty even across rad. Used the backflush system to get everything out. Then the bummer...

Still have the oil slick on the otherwise clean, backflushed water. I know this isn't good. I was hoping(?!) it was a leak in the trans cooler but I took off the top hose and got the water from there and sure enough, oil slick. So......how do I know if it's head gasket or just(HA!) the intake manifold procedure. I've read pressure test. Does that determine my next move? Is that something I can do? Does anybody have a link to procedures?

Thanks for your help.

What kind of oil flow did you get through the oil cooler in the radiator tank? Is it flowing okay?

You must identify if this oil slick is fom the Dex cool or trans fluid.

Went out and checked the flow....about a quart in 10 seconds! Faster than I thought. I didn't check output all the way to trans, only from rad....Fluid doesn't look great...

How can I tell if the sheen is dex or oil? It's just a sheen and there is no color. Is there a test solution that would help idendify it as oil? Seems I read of an additive or something that changes color if there's oil present. Been reading a lot though.

If you look in the radiator is there any clogging of cores? Have you been adding trans fluid? Usually you have to send a sample to a lab for analysis, there are many around the country that do this for about 30.00, but that may not be necessary.

When I started the flush without the thermostat in, the fluid dropped a bit and there was a lot of flow through the cores...at least the top 4 or 5 rows. Trans fluid level has been maintained since I have a slight leak. Maybe a half qt every couple of months. The trans fluid sample I drained yesterday looked really dark but didn't smell burnt to me. Didn't look fresh but it really didn't after I drained and filled last summer either.

Do you think there's any way the the trans fluid is so hot that is causes the rad to boil over right at the trans cooler? Cap is weak/leaking...Coolant wasn't in ideal condition...

While I got you reading....if you do a search for "Code PO1870" in LeSabre forum, that thread describes what my tranny is doing. Whaddya think about drain and fill with Dextron VI and Lubeguard...get a new cap...put fresh coolant in...maybe some sealant(advise) to find the oil slick issue....and order the intake manifold redo kits for another day? Dang, that was a long question!

Sounds like a plan, there is no way the trans fluid would get hot enough to cause the engine to run hot, That would probably cause a complete trans failure.