When I'm making a new prom and notice some lope. I watch the RPM MAP or MAF position and map out the area around the idle to be the same fuel and spark settings. No more problems! If you check the area around 600 RPM 35kPa you would see that each block around the 600 is different. That can cause havick. If you have checked tune-up parts, computer sensors, Base settings are correct, that would leave the PROM.

Also is mixture, Air / Fuel correct?

Add little propane to make richer?
Make a vacuum leak (brake vacuum hose with thumb covering it) Leaner!
One or the other should make the idle better!
If not:
1. egr stuck on at idle!
2. miss fire.
3. mechanical
Do cylinder balance test.
Buy 1 foot of windshield vacuum hose (only).
Has carbon in it.
The size of spark terminal end and cut about 1 1/8 inchs
make 8 and put between the plug wire and the cap.
Start the car and let idle.
Now take an insulated wire and ground one end to the block.
For the weak of Heart use insulated pliers and leather gloves.
Touch rubber one at a time. They should all drop the same!
Sometimes you need to unplug the O2 to stay in open-loop.
Have Fun!

 

Thanks, Avantihall, for the suggestions. I'm going to look for vacuum leaks first, then clean the IAC assembly, then look for loose grounds, then hook up my scantool, then....

 

It sounds like it could be a stuck EGR valve. Try disconnecting it and plugging the vacuum line, temporarily. They can be carefully disassembled and cleaned and the seats lapped if the diaphragm is OK.
I never had a problem with my 87 with a 180 thermostat, but all cars are a little different aren't they?. I didn't like the 160 thermostat in the car when I bought it, ran rough until it was really warm and hurt mileage. My IAC also failed and caused idle problems.
You might want to also clean the throttle body/throttle plates.
Dave

 

Dave, thanks for the tip. I'll add it to the (growing) list of things to check, when the snow sucks and I'm stuck inside for a weekend.

 

Hello, our 89 L98 (stock 80k) was exhibiting a maddening hunting/surging at idle. RPM would randomly hunt from 400 (near stall) to 1500, not too bad when cold but worsened once in closed loop. My son was driving it like that for a few days before I witnessed it and started the search for cause/fix. Ran the ALDL monitor on a homemade serial interface to my laptop via Datamaster and verified all sensors intact and reading, could not see a source for problem, but all data fluctuated more or less in sync with RPM variations.
Did the following to troubleshoot:
1)Plugs and wires, distributor pickup coil, cap and rotor replaced.
2)Pulled thottle body, cleaned IAC, passages, set up minimum idle, TPS etc.
3)Ohmed out injectors, found one @2ohms, rest app. 16.
4}Did a crude injector "balance test" by disconnecting one at a time, inconclusive results, no real change with any one injector.
5}Checked fuel rail pressure, was 39psi at idle.
6}Pressure decay test, found pressure dropping from 40 to 10psi within 10 minutes, whether I pinched off the return or supply line or not.
7)checked using various techniques for vacuum leaks, verified EGR ok.

From the above I decided that I had at least one and possibly several injectors failing. After much research and confusion, decided to replace the stock 22@injectors with "25#" LS1 injectors from SLP ($100 price was the #1 factor), and pull the intake and fix some small coolant leaks at the same time.

Issues doing the swap: had to set up to do Prom editing to set up fuel pressure constant for larger injectors, LS1 injectors were shorter and I had to griind off .2" for my fuel rail mounting pedestals on the intake. Not sure this was absolutely needed, but installing in stock position would have the inake manifold side orings barely sealing so I lowered the fuel rail for peace of mind. Also ran very rich at 25# inj. constant, changed to 28#. NOTE: if I had to do the job again I would have looked for 22# mustang injectors which wouldn't have required either of these two mods....

Result: Idles perfect, MUCH smoother operation throughout operating range. Still haven't run data log to check but from plugs and MPG looks very good. FWIW, this was my first attempt to work on an Domestic car and I would never have been able to proceed without this forum and also Thirdgen.org TPI tech forums. Sorry this post is so long, but when I was searching for info/help with this I was glad to read all I could.

 

Thanks, jccyr , for the war story. I'm hoping my journey will be somewhat shorter!

 

My 87 does the same thing ... idle will surge from 600 to 1000 in gear (automatic). I have four codes locked, EGR, O2 sensor, MAF, and MAF burnoff relay. I'll be checking these as well as all of the above.

 

With that many codes you may have an ECM going bad, especially if intermittent. Pull the ECM fuse to reset, or disconnect battery to clear trouble codes, then see if they all come back immediately. I had all the same codes, replaced relays, 02, MAF still had problem intermittently, ended up being a bad ECM.

 

Thanks .... I am suspecting that as well. Sometimes it will clear on its own .... and after pulling the ECM fuse they all clear then return. Other little gremlins are unsteady average fuel mileage reading (usually 20 ish but will drop to 12.6 occasionally ... when switching over to "instantaneous" it reads "0". Water temp also starts at 299 (open) then will come down to 185-195 range. Not an overheating prob as the oil temp reads steady and correct for conditions.

I spoke with a former C4 owner at a car show last weekend (who was also a former Chevy service manager) ... he had a relay issue causing this similar condition. When checking the relays I can hear the burnoff relay function (it clicks off about 2 seconds after I shut off the key) ... but couldn't tell if the MAF power relay is functioning. Tomorrow I break out the DMM and start troubleshooting.

BTW ... this condition has existed for several years with previous owner .. and I have driven it 8000 miles through three states since I bought it in June. Agravating but consistent. I've played the faulty ECM game with a string of S10s from 86 through 98.

In spite of it all .... still a damn awesome car! Once you put your foot in it you plant into the seat and it gets up and goes. Will even chirp the tires on dry pavement and 90MPH is there before you know it. So the only engine management issue is at low RPM. Under acceleration it's pretty much on target.

 

Now I would almost bet my life that it is the ECM, along with the above problems, my INSTANT/AVE Fuel readout also started going haywire too, thats when I got a new ECM and it cured everything.

I got mine at Carquest, it was a reman from Canada, one year warranty, there may be better warranties elsewhere, not sure.

 

Sounds like a done deal .... I'll go the ECM route. Based on my dealings with S10 ECMs it is probably unreasonable to expect that the Corvettes got better ones than the rest of the line.

If I had my time machine I'd go back to 67 and get my cousin's new big block fastback that he bought between 'Nam tours. Can you imagine the rush of being 13 and riding in that thing! (well ... you're all Vette lovers, guess you CAN imagine it!). The vision of hurtling down the road, feeling the front end get light and the body flex as he banged second .... after college I bought a 65 Goat and tried to recreate the feeling but it just wasn't the same. Not that it was a BAD feeling in the GTO either

 

Went out today ... too cold to futz with the relays, MAF, and ECM so I did some soft repairs like fixing the mangled glove box catch. SES still lit but idling lots better. Went to pull into traffic and lit the tires up and snatched her sideways. I guess if you pamper them by fixing little things they resopnd better.

Maybe if I lay a dozen roses on the dash the ECM will fix itself ... LOL.

Tomorrow I will take her out on the open road for a 200 mile drive to a friend's house to swap some neat old vintage license plates and get some new ones for the garage wall. Maybe hauling some classic tin (and hauling a$$ a bit to) she'll perk up. Sunday I will check all of the above items.

I'm wondering if I have a low voltage issue adding to the mix. I will clean and check connections and posts too. Any car that spins tires that forcefully and effortlesslty seems pretty close to on target.

Save the wave ....

Tom

 

Today's update ... MAF was clean and intact, connectors clean with no corrosion, power and burnoff relays actuate .... no vacuum leaks detected ... still has 13, 32,34,36 codes. Cleaned MAF and TB for good measure, pulled ECM fuse to reset, codes came right back. Still runs great other than erratic idle. Dropped her into 1st (4spd auto) and punched it .... throw into seat acceleration. Seems like all that is wrong is the ghost codes. Haven't checked O2 sensor (13 code) or EGR (32 code) but based on posts above and the overall condition/operation of the MAF and relays I don't suspect I'll find an issue.

Trip to advance for new ECM .... will swap out tomorrow and test further.

I know I got the neighbor's attention rumbling down the street at 3 grand, then backing off so the exhaust rumbles just right (not speeding ... in first gear ... perfectly normal). Well ... perfectly normal for us! Far cry from the little VWs I usually drive (although the cammed up, dual carb one sounds pretty mean on it's own right. 100HP in a 1700 pound car works pretty good!)

 

Swapped out the ECM today ... quite a job for someone 6'2" tall to cram under the dash and pull that thing ... all codes cleared and TREMENDOUS horsepower! Idle hunting cured (1100 RPM fast idle ... slowed to 800 when warmed up ... 600 in gear. The flaky display is cured (except for the temp reading which is a bad sensor). All codes cleared for two start cycles .... drove it to Advance to return the ECM core ... then SES came on for third start to come home. Came home (still running strong) and pulled the codes ... now only 36 (MAF Burnoff Relay). Tomorrow get new relay and should be in business!!! The relay is clicking on and off but evidently has a bad set of contacts.

I never realize how badly she was running before ... it was fast and all ... but now I can definitely tell a difference.

Now I have to bolt the dash back together!