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Have a strange one here.. Car, 1995 wagon Lt1, was running fine before this.. A/C compressor went out, installed new compressor and hoses and all related needs, replaced all hoses since I was in there, changed thermostat, and belts. Put everything together, changed spark plugs (NGK) filled it up with water/AF started it, ran fine, no leaks, bled the little air valve on the thermostat housing a few times. Have not plugged in the AC compressor yet. (still need to evacuate it.. etc). Car, has 5.7 with heavy duty cooling, 1 mech fan and 1 elec fan plus various other towing package items. Problem is, as long as you are driving it, all is normal. As soon as you let it idle for a minute or so, the engine kind of stumbles and the elec fan comes on and the check eng light comes on.. If you drive it for a few seconds, the light goes out and the fan turns off, or you can just kill it and start it and all is fine for about a minute and it does it again.. Temp gauge never shows over halfway, as it always has and hoses never shows over 150 or so with a thermal gun.. Smog pump fuse is good. Thermostat seems to be working right also.. Irks me that GM engineers made it near impossible to pull the error codes (paper clip trick does not work on this model) Normal OBD-1 do ot work on it either . Dealer said they would pull the code for $150, and before you say it, Autozone nor O'Reilly's can do it. '94, '95 5.7 are some special sort of bird. Any help will be appreciated...
 

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Get that check engine light read. .. Better still, just take it to your Chevy dealer and let them fix it.
 

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The CEL means that the code was thrown that will probably point you directly to the problem. Get the proper cable to do your own diagnostics, download (free) eehack and go to town - or let the dealer sort it out. This cable is well-known to be the best one, and most reliable one out there.
 
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Thanks.. No dealer ripoffs for me.. I don't mind spending a few bucks to get the proper tools.. I just couldn't find any obd1 stuff to work on my '95 caprice, but it looks like you have one so it must work.. Does your vin have the P for the eighth number? I will check it out and let you know..
 

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That is the cable I use and it works fine
 

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A and B are power and ground, M is the data.
 

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Nope
A and B are power and ground
B is field service enable.
You might see a voltage reading, dont know, but it ain't power!

A ground
M data

As other posters have stated, these are the 2 connections needed to communicate with the 94-95 PCM
20200112_103753.jpg
 

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Yes, these cars need an enhanced OBDI software scan (two years only, as it was obsoleted by OBDII in '96). Some scanners can interface with these computers, but most cannot. If you are decent with a computer, then the best bet is the recommendations already made (get your own USB cable and use a laptop).

Albeit if touching a computer scares you, then I can dig through my notes on a few off-the-shelf scanners that should work. Of course there have been previous discussions about this subject on this forum.

Once you scan the code, post it here. There are a few codes that are misleading (ie: knock sensors which sometimes will report as an ignition system fault). So we can help you do the proper diagnostics.

In fact, the symptoms you report sounds like a knock sensor issue I chased for a while on a '94. Replacing the A/C system parts and hoses puts you at risk of disturbing the knock sensor and/or the wiring to it on the passenger side. In my experience, simply unplugging the knock sensor can be enough to cause intermittent connection with these 25 year old connectors. And worse, just bumping the connector with a tool or tugging the wire can cause a crack in the wire or the sensor housing itself. And then this will cause a random knock sensor code...often at idle for some reason. My theory is that once you start driving, the vibration can be enough to make the connection again and turn off the light. Not saying this is your issue, but it happened to me and I figured I'd share my experience.

I'm also wondering if you got a little water in the Opti during your cooling system maintenance if you spilled any. That can cause random CEL problems too.

Get the code and report back. Otherwise we are just guessing.
 

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Discussion Starter #11
Just an update.. Diags report both a 44 and 64 error, which is a lean condition on both O2 sensors and since they both appear simultaneously, it means something upstream is amiss.. It points to the MAF sensor.. The only wire I unplugged in the water hose replacements was the one on the rubber boot on the intake.. Haven't had time to mess with it yet.. Will let you know what I find.. Thanks for all your help so far..
 

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....installed new compressor and hoses and all related needs
..... replaced all hoses since I was in there,
....changed thermostat, and belts.
.....changed spark plugs

.....runs fine, but let it idle for a minute or so, the engine kind of stumbles and the elec fan comes on and the check eng light comes on.. If you drive it for a few seconds, the light goes out and the fan turns off, or you can just kill it and start it and all is fine for about a minute and it does it again.. ....
....
I've gone through posts several times, and some guys touched on possibility of problems stemming from stuff unrelated directly with your work (KS, MAF). From further out in left field I'll volunteer IMMEDIATELY thinking to myself (based on personal experience mind you), "That is one ****pot full of work not to have risked inadvertently dislodged/broked something nearby." Here's some 'out theres':

1. ECT: I read over the years all the phantom trouble this thing causes ala hard start, no start, crappy running, dying.... I heard you can force both fans on by pulling it, so to diagnose a recent never-running fan I did that - and the car started missing and bucking but then evened out. I tried starting it once and it took awhile to turnover, then ran crappy, but evened out in a bit and got better as I revved. It's near the TS and all those hoses, and the wiring/connector seem to go bad after a couple decades. Use that for what you may.

2. Plugs: Changing those out requires hands and elbows over so much of the engine packed with other wiring and components that it's a target rich environment for collateral damage ;)

3. AC: All that work is sure to 'leave a mark' on the entire PS sensors and wiring (good call on the KS connection).

What I've done in past sim. situation(s):
Plug wires. You touched them, and they don't like that after 20 years and/or 100k+ miles. Weak/cracked so they carry badly at lower idle v. higher alternator power/current/amps. Consider compounding affect on old/weak IC and ICM (if not already mentioned). If you did plugs, then you more than need wires too.

ECT: Pull it running, and also trying to start, and add any symptoms to your elimination list.

Vacuum hoses/wiring /connectors: I've split a hose on the opposite end of replacing a solenoid, and found it after a week of grief. A guy recently reported back here that he found an accidentally disconnected wire to something on the intake after working around his WP.

For therapy dealing with a problem I'll take an hour to inspect every single connector and treat it to some dielec. grease, just to show I care and to eyeball for anything I don't like the looks of.

Hoses: I yank each end off trying to break it. Some get snipped for fresh end, but the the weak or dying are disposed of without mercy and replaced.

You got smart heads here posting, and you'll find the fix soon. Be sure to report back when you find the culprit.
 

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Re what 96 Black just said:
The H2Opump temp sensor's wiring is some of the flimsiest ever. When disconnected (intentionally, or while the wiring is intermittently not doing its job), it runs very rich, but it may still run.
25 year old plastic and rubber, and wiring that was too thin to begin with …
 

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Discussion Starter #16
All valid possibilities, just keep this in mind.. The car starts fine, idles perfect, smooth, no odd noises, when started, runs perfect. It has all 260 horses at my beck and call.. Smooth, not one miss, temp gauge is right where it should be, when you stop, it idles perfect for about a minute, smooth as can be then after about that minute, it stumbles like it is running out of gas, it has a new fuel pump BTW, (not a contributor to the problem as it has been on there for months with no problem) then the CEL comes on and the fan kicks on. Funny thing is, while it is still idling with the light is on, you can put it in drive and drive it for about 15 seconds, CEL goes out and every thing returns to normal so it is probably a vacuum leak or something of the sort.. and it never did this before replacing the hoses and AC parts. I am doubting the AC replacement has any bearing on it. FYI, This is more of a project car than a necessity but I will get it fixed. .
 

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We got all that. Along lines of parts having consequence on performance (or even function) of other parts; try plugging in the AC comp. and pull whatever the first fuse is which powers it in case the PCM needs to sense that it's there - even if not running. I know, a stretch, but these car's sensors are connected together in even stranger ways than that.

And you're 'ok all the time except idle' also sound like IAC or TPS issues too. Did anyone say, "EGR" yet? Is there alot of grime and oil on the bellhousing or down the timing cover = bad intake caulking.
 

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Discussion Starter #18
No excessive grime.. EGR could a possibility, never has been changed..... I will get it in and look at all of that when I get a chance.. This is just one of many projects I have going and since it is a small issue, it may be a bit before I dig into it again.. . But, I will report on my progress.. Thanks again...
 

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OK.. Found it.. Rotten vacuum line under the Home plate.. Removed the H.P. and I could hear it..Runs like new now.. Thanks for all the suggestions..
 

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Tkx for circling back. That makes 35,862,504 times a simple hose, wire, connector or ground has caused dirty rat bastard operation.
 
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