I need help again...Mini Van getting 10mpg

[Bigbuds1]

I have a '91 Olds Silhouette 3.1 I drive to work, I have replaced the TPS and MAP but it still wants to run the same and surge and idle at 2500. Sometimes it runs fine, others it will run good for about 5 minutes the suddenly revs up to 2500 and stays there. I think I have checked all the vacuum lines, they seem fine, but If I get it to run at normal idle tomorrow I will spray it down with WD40 and check them that way. Can the ECU intermittently act up like this? I don't know where else to look. ??? ???

Thanks, Buddy

[SuperBuickGuy]

Re: I need help again...Mini Van getting 10mpg

have you checked the codes?

[Bigbuds1]

Re: I need help again...Mini Van getting 10mpg

Aaron, yes, 15 (coolant sensor) and 34 (map). But now it only shows 15. I don't see how the coolant would effect start up.

[squirrel]

Re: I need help again...Mini Van getting 10mpg

Buy an old 4x4 suburban, they get 13

if it thinks it's still stone cold, it will want to rev it up to get it to warm up faster?

[sedandelivery55]

Re: I need help again...Mini Van getting 10mpg

Buddy, the coolant sensor can affect idle and start up. The one on my Chevy pickup went bad and the truck would not idle and run like crap. When I finally found a good one, the first two I bought wouldn't work( cheap ass china crap) then went to the dealer and paid twice as much but never had a problem after that.

[redragtop]

Re: I need help again...Mini Van getting 10mpg

Buddy, You need to get data (not just code) on coolant sensor. If it intermittently "thinks" it's cold(-40), it will richen up the mixture(choke) and raise RPM's and suck fuel like crazy. Fairly common problem. Intake air temp(IAT) and coolant temp should read same at initial start up. Good Luck

[milner351]

Re: I need help again...Mini Van getting 10mpg

I had a similar issue on a v6 ranger pickup - mileage went to crap for no apparent reason.

The thermostat was stuck open.

I would try to get a temperature reading on the coolant - and compare it to what the temp sensor is reading....

sounds like a bad temp sensor to me - but I'd make sure the thermostat is OK as well.

[SpiderGearsMan]

Re: I need help again...Mini Van getting 10mpg

91 ? check to see if the throttle body is clogged
if the coolant sensor goes bad it shows -20 degrees and dumps fuel , also the connector gets gummed up with coolant
the map codes can be from poor engine vacuum from the overrich , check cam timing too
pickup coils go bad in the distributor

[dieselgeek]

Re: I need help again...Mini Van getting 10mpg

Aaron, yes, 15 (coolant sensor) and 34 (map). But now it only shows 15. I don't see how the coolant would effect start up.

Spidey hit it before I did...

if the CLT sensor goes bad, the computer thinks the engine is running at insanely cold temperatures. Enrichments on GM cars are as much as 150-200% in cold weather, I think you found your issue.

The computer will also use the IAC (idle control) valve to increase RPM if it thinks the engine is that cold. BUT - it can also be a huge intake air leak. I'd suggest replacing the CLT sensor (make sure you get the right one, there will be TWO on your engine) first, then see if the idle problem goes away. I bet the MPG issues is fixed with the replaced CLT sensor. It might also be a bad connector/pins on the sensor or harness, so keep an eye out for that as well. Those CLT sensors do not die easily.

[milner351]

Re: I need help again...Mini Van getting 10mpg

Don't remember for sure on this - but don't you have to be careful with the threads on a CLT sensor?

The ground is through the threads - I remember someone telling me to make sure I didn't use sealant on the threads....
???

[TheSilverBuick]

Re: I need help again...Mini Van getting 10mpg

Don't remember for sure on this - but don't you have to be careful with the threads on a CLT sensor?

The ground is through the threads - I remember someone telling me to make sure I didn't use sealant on the threads....
???

ECM coolant temp sensors have a ground wire. The old idiot light or about all dash gauge ones are grounded through the threads. I've always used thread sealer, and my stock gauge reads normally. Even better so now through the aluminum....

[squirrel]

Re: I need help again...Mini Van getting 10mpg

I never could figure out how sealer could insulate a pipe thread? every time I unscrew anything with a pipe thread, there's lots of shiny metal


But yeah, computer coolant sensors have more than one wire....

[SpiderGearsMan]

Re: I need help again...Mini Van getting 10mpg

yellow and black - they act like a pipe to fill the ecm with coolant on the pre metri pack connectors

[Bigbuds1]

Re: I need help again...Mini Van getting 10mpg

Thanks again for all the help everyone, I will check into another coolant sensor and IAC. What I will do first is get the part numbers then cross reference them to the '91 Firebird 3.1 I have out back (under a foot of snow), although it is MPFI not TB. Maybe I can try some parts I already have? I'm trying not to waste $$ because my LT1 short block is almost done. :o It did seem to run fine on the way home from work today though, still hard to start. I work all weekend so Monday will be my next chance to wrench. I just hope it don't leave me stranded...

[dieselgeek]

Re: I need help again...Mini Van getting 10mpg

Thanks again for all the help everyone, I will check into another coolant sensor and IAC. What I will do first is get the part numbers then cross reference them to the '91 Firebird 3.1 I have out back (under a foot of snow), although it is MPFI not TB. Maybe I can try some parts I already have? I'm trying not to waste $$ because my LT1 short block is almost done. :o It did seem to run fine on the way home from work today though, still hard to start. I work all weekend so Monday will be my next chance to wrench. I just hope it don't leave me stranded...

Startup fuel is based almost entirely on coolant temp.

The sensor in nearly ALL 1990's era Gm cars is identical - pull the one out of your other car and give it a shot!

-Scott