Wading in over my head

[redfury]

Need some help to make sure I don't throw money down a rabbit hole unnecessarily.

I just bought a 1998 Astro with a 5.7 Vortech from a Savanna van. The PO claimed the motor was from a 99, but the PCM in the van seems to be from a 97 truck. The Black box serv.no. is 16244210.

Here's my problem...The truck starts right up, drives through the RPM range just fine, but it idles at 1000-1500 rpm! It'll eventually drop to about 750, but if I touch the gas again, it goes back up and stays there for an undermined amount of time. Also, the tranny doesn't shift like it should, the other day it slipped like crazy ( very cold out ) and once I had driven a few miles and stopped for coffee and came out, it was shifting again. The 1-2 shift is decent, the 3rd gear seems soft and it drops into 4th heavily. I don't think it's a tranny issue...he said it was rebuilt 10k ago and there is good pressure, it's solid when it's taken out of Park and put into gear, forward and reverse.

I've been doing a lot of reading here and it seems to me that there might be a harness issue since there supposedly was a change from 97 to 98? I had a p1406 code with freeze frame data stored, as well as p0122, p0327 and p0420-30 codes set originally, but when I removed the codes with my scanner, only the EGR Pintle position code showed up again. I pulled the EGR and replaced it as the plunger seemed to stick a little ( found a nice one in the local yard that operated smoothly and in much better physical condition with a newish looking gasket under it ( assuming it was a replacement part not much before the demise of the van it was on )

I replaced the TPS as my scanner was showing 0 - 1.6 fluctuation and pulled the IAC valve to check for blockage/carbon and operation. I plugged the IAC into the harness and turned the key on to check operation, the motor moves the valve smoothly, so I don't think the IAC is causing the high idle. My next step is to do a vacuum test now that I've verified the other components.

HOWEVER, I'm stuck on the idea that this is a computer issue. The local go fast shop wants $275 to do an off dyno tune. He says as long as it has a v8 PCM, he can do it. Before I even think of shelling out that kind of money, I want to make sure I'm providing a good canvas on which to tune. I don't have the ability right now to go crazy and do a loom switch to a 411 PCM ( which I can get for 50 bucks at the local yard ), but I'm wondering about the 97 that is in there now. I wonder if it isn't cause for some of the problems there is with general driveability and if I would be better served updated the PCM to a 98-00 PCM. He claimed to have just done the extra wiring for the extra injectors, so I assume he didn't do a wiring swap with the motor, just used the existing from the 4.3. Should I expect a 98-00 PCM to be more plug and play with the motor/tranny or should I just buck up and go see the speed shop?

My driveability isn't what it should be, and I'm getting 12mpg. I know this set up is good for much better MPG.

Also, the transfer case is a pushbutton 4x4, no AWD.

I'd certainly appreciate anyone who has any advice...especially if you went through the trouble to read my little diatribe!

[Taz]

Hello redfury,

From your description it sounds like you have too many variables in play to make any kind of reasonable decision on how to move forward. The first thing I would suggest is confirming the accuracy of the wiring harness / PCM pinouts. This won’t cost you any money, just a few hours of your time.

Get a list of the PCM pinouts for a 1997 5.7 L Vortec truck, and confirm each wire in the harness. I use a digital voltmeter set to “continuity” with an audible tone. Unplug the 4 connectors from the PCM (blue / red / clear / black), then check each pin one at a time – with the corresponding contact at the sensor / connector.

You listed the following DTCs as being active:

P1406 - EGR pintle position
P0122 - TPS
P0327 - knock sensor bank 1
P0420 - catalyst efficiency bank 1
P0430 - catalyst efficiency bank 2

Odds are some of these will be caused by wiring issues. Confirming the wiring harness integrity as described above will assist in this diagnosis, as will confirming / cleaning all grounds.

If you’re only getting 12 MPG, then raw fuel is going out the tailpipe. This might be the cause, or at least a contributing factor, of the P0420 and P0430 DTCs being set.

Also check for vacuum leaks, coolant in the combustion chamber, and verify the distributor has been properly installed / adjusted – if it’s off a tooth or two it will never run right.

I would recommend all of the above before considering a custom tune.

If you can, confirm the tune in the PCM is a “stock” 5.7 L Vortec tune.

Lastly, a “0411” swap is very straight forward, and requires only an LS1-B PCM and connectors. The existing wiring harness is used - with generally no cutting or splicing required.


Regards,
Taz

[redfury]

Yes, I agree that a full integrity check is in order. I think I confirmed my belief that it is not a vacuum problem causing the high idle tonight though. I just got back from a test drive. I reinstalled the IAC and reset it using the key on ( 10 sec ) key off ( 5 sec ) and cleared the codes. Idles great! No codes came up during a round the block shakedown ( quicky ), but I did notice something that needs to be addressed.

The idle is right where it should be, BUT...when I put it in gear, the idle automatically raises to over 1000rpm, which in this snow, overpowers the ABS and the only way to stop efficiently is to put the van in Neutral ( idles right back down ).

So, I've got something going on in relation to the transmission and the PCM telling the engine to adjust for a higher load or ???

Never ran into this before. Thoughts?

Also, with the 4x4 conversion, I believe he added the switchbox to the underside of the dash, but it seems to be held in place with zip ties and may have a questionable ground, as while I was working, I moved it and could here relays switching. I'm going to have to ground that properly for sure.

[Taz]

There are Desired Idle Speeds (B1200 in your tune – I think) for:

• in gear A/C off
• in gear A/C on
• in P/N A/C off
• in P/N A/C on

The idle speed will increase a little when you put it in gear - at normal operating temperature.. However, when the engine is cold (ECT of -4 degrees C or less) the in gear idle may be over 1000 RPM. If the ECT sensor is faulty, or improperly wired, the PCM may “think” the engine is cold all of the time. This may also be contributory to the poor fuel economy.


Regards,
Taz

[redfury]

Do these engines have two coolant sensors? The one on the front of the motor looks like it had been replaced recently, the brass is still shiny and the thread tape is still white. I know that sometimes there are two, one of the gauge, one for the PCM.

[redfury]

Keep in mind, I have a cheap Actron Scanner that can tell me some live data, but not much more than that. I can see coolant temp, TPS, calc load, etc, but nothing more than that. When I drove off, the engine was up to temp, I know that much because it had been idling for 15 minutes before I took it out for the road test.

[Taz]

I don’t do any work on the Vortec (Gen I based) engine line. Whether you have one 2 wire coolant temp sensor, one 3 wire coolant temp sensor, or 2 sensors (one for the gauge, and one for PCM) – I have no idea.

A good quality manual will assist you greatly. Helm is the only source for GM OEM type manuals. Given the vehicle is a conversion, which you didn’t perform – a good manual will be an invaluable resource.


Regards,
Taz

[redfury]

Where does one get a manual from HELM? Never heard of them actually. All I ever end up with are the Haynes/Chilton manuals, which are essentially "cars for dummies" and I hate them with a passion. I'll take a good shop manual, I had a great one for my old 1965 Plymouth Fury III...showed you how to do everything...loved it, want one for everything I own. Reference manuals are bibles to me. Give me a fish, feed me for a day, Show me where to buy the manual, I can fix this shit myself.

[Taz]

Online at www.helminc.com Select the Vehicle Owner / Do-It-Yourself / Professionals link – then search by vehicle. Guessing you will need “GMT97CK” - $135.

You could also try www.alldatadiy.com – they charge $16.95 per year or $29.95 for 5 years. I have never used this service – but reviews generally seem positive.


Regards,
Taz

[redfury]

We've got Alldata at my work, it's a pretty good site and comprehensive. I might just have to drop the 30 bucks on it, might be worth it in this case.