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Old 02-25-2011, 04:05 PM   #1
B1rdman
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87 Chevy 350 TBI low Idle issue

I have a 1987 Chevy Silverado, 350 or 5.7 engine with TBI. I have had the truck for 17 years and it has about 171 k miles on it

Cold it starts up and runs fine. It even runs fine when warm except when you come to a stop sign or light and have to sit at it for more that 10 seconds where it dies. It will die also when you put it in reverse and turn the steering wheel while at idle. It seems to be like a low idle issue. It starts right back up with no problems and if you can catch it by putting it into neutral or keeping one foot on the brake and giving it a little gas it does not die when coming to a stop.

I actually have had this problem with the truck for several years but its dying has gotten a little more frequent of late so I am trying to find out for once and all what is the matter. Even with the miles it has been a very good truck.

I have actually taken it in to a couple of very good shops over the years for them to put it on the scanner and other test equipment and they could not find any problem.

There has been no codes at any time on it.

Using the diag mode with the key on I watched the IAC relearn process through the top of the throttle body.

I Changed the MAP out with a known good one.

The egr seem properly seated with no leaks, there is no rough or high idle

Checked the CTS with millimeter for resistance at high and low temp. 70 degree it showed 3900 ohms and at 210 it was right at 200 ohms.

Checked for vacuum leaks with carb/TBI cleaner around the base of TBI. Checked all vacuum lines

Injectors are not leaking and have a nice conical patterns.

Sensor voltage is right at 4.9 volts. The standard is 5 volts plus or minus .5 volt. I cleaned and tested sensor grounds.

The truck just made a 2000 mile trip from Wa state to Texas with a load in back and had no problems except the occasional dying issue when leaving the interstate. Throttle never felt starved and could maintain 70 mph easy with a lot of throttle response. So I do not think this is a low fuel pressure issue.

The only thing I have not been able to do it check the timing since I do not own a light. I have a friend who has one who is coming over this weekend with it so we can see if it might have a timing or distributor issue. He thinks it’s a wore out distributor. So with the computer disconnected if it shows 0 degrees TDC and a steady timing mark, I do not think it’s the distributor actually causing it to die. Normally a wore out distributor causes a rough idle unless the module itself might be causing a low idle.

The only things left I have not checked because I can not afford to throw parts at it is the distributor, TBI, Computer. I thought the TCC might be causing it, but I was able to get it to die just sitting in my driveway after getting it to operating temp, by putting it into gear and turning the steering wheel while at idle.

Since I do have a way to monitor the rpm I have not done the minimum air adjust. Since it is actually running good except for the idle issue I do not want to mess with something that might make it less drive able right now.

I guess I am looking for anything that I might have missed here that will not cost me an arm or leg to test it or some other indications that I might be missing.
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Old 02-25-2011, 04:08 PM   #2
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Re: 87 Chevy 350 TBI low Idle issue

Where is your TPS voltage set at? Should be .5 @ idle. Which sensor is 4.9V?
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Old 02-25-2011, 04:23 PM   #3
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Re: 87 Chevy 350 TBI low Idle issue

It sounds like an IAC problem to me.

The IAC (Idle Air Control) controls the air passing through a throttle pate bypass passage.

The IAC (and computer) control the idle speed.
The IAC (and computer) is what gives the engine a “fast idle” speed when started cold.
The IAC also supplies more air to compensate for the extra load when the A/C is on. If you have A/C and turn it on while listening carefully you should be able to here the IAC controlling the idle as the A/C clutch engages and disengages.

IAC passages can get plugged up with carbon or muck. The IAC units get erratic as they get old and stick. Use care when cleaning the IAC. The IAC should not be cleaned with carburetor cleaner or other harsh chemicals. If the chemicals get down inside the IAC, it will damage it.

To clean the IAC, you should remove it, and use a wet rag to wipe the tip of the plunger, while holding it with the plunger pointing DOWN. Be careful, the IAC will break easily. It is aluminum. Clean the passages in the TBI unit while you have the IAC removed.

I do not suggest unscrewing the pintle (plunger). If you don’t get in back in far enough, the IAC can get damaged if the “pintle” (plunger) bottoms out in the TBI, when you screw the IAC back in.

Usually the IAC does not go bad electrically, but it is common for them to stick and stop moving.

The IAC is a stepper motor and uses short pulses of electricity on the wires to move its position.

If the IAC sticks as does not move as it should for each pulse, the IAC count will get off because the computer does not know the actual position of the IAC, but the ECM tracks the commands it has sent to the IAC and assumes it has moved every time it has been sent a pulse.

Questions:
When the engine is started, does the engine have a fast idle that slows down as it runs? The IAC (idle air control) is what gives the fast idle speed when started cold.

Does the engine start well without the gas pedal being pushed down at all?
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Old 02-26-2011, 08:19 AM   #4
B1rdman
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Re: 87 Chevy 350 TBI low Idle issue

Here let me answer a couple of questions.

I did not check the voltage on the TPS on this round of troubleshooting but I did a while back and so did the shops I took the truck to to see if a scanner would find anything. Supposedly the voltage then was nominal and correct and the TPS show smooth operation from idle to WOT. I will test the voltage again when I check my timing and distributor later.

I used the reference voltage on the MAP sensor to check the source voltage for the sensors as applied from the regulator circuit on the computer. It showed within specs as listed.

My initial thoughts and on more than one occasion that this was an IAC problem. However I pulled the IAC and the Pintel was not gummed up but I sprayed cleaner into the area in which the pintel goes. I did not spray any cleaner into the IAC. I checked to make sure that the pintel was exactly 1 and 1/8th of an inch out and reinstalled it. I jumpered and pins A and B in the diag connector and got on top of the engine to observe the IAC relearn process through the top of the TBI while I had a helper turn the ignition switch to on w/o starting it. I observed proper mechanical operation during the relearn process. According to everything I have read about the IAC, this is an indication that it works properly both mechanically and electrically. If it is an IAC problem then its the controller to the IAC which is the computer, IMHO.

When cold the engine runs at a slightly higher speed and slows down as it gets warm. It seems normal when in park and does not run rough anytime at idle or at speed. It makes me think it does not get the boost in idle by a 100 or so rpm when in gear. I have not tried the AC to see if it kicks the rpm up. Living in the Pacific Northwest I have not had to use my AC in a long time. I will have to check that out also.

If the temperature is above about 40 degrees, all I have to do to start the truck is turn the ignition on and wait a few seconds(5 or 6) for the fuel pump to charge the system then without touching the throttle, the trucks starts right up. If its colder than 40 then it depends how much colder on how much throttle I have to give to get it started. Around zero I have to work the throttle a little to get it started and keep it running for the first few seconds. But it has always started this way since I bought the truck. I have never had a problem starting it. In fact it has been the easiest starting vehicle I have ever owned.

I will get back when I double check all of these things later when I get the timing light to check the timing.
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Old 02-26-2011, 09:16 AM   #5
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Re: 87 Chevy 350 TBI low Idle issue

You have a much better understand of the IAC then most people, which is great.

Quote:
However I pulled the IAC and the Pintel was not gummed up but I sprayed cleaner into the area in which the pintel goes.
That is very risky.
Being you watched for and saw movement of the IAC pintle, the IAC is still working somewhat but it may not be moving as much as it should.

The IAC my be moving for some of the pulses sent to it but not all always moving for every pulse sent.

You can see the IAC count with many scan tools. When started cold it should show about a 150 count. If it does show a count of 150 and does not idle fast you should suspect the IAC is not moving as far as it should.

AT normal hot idle the IAC count should be under 50. I like to see them at 15-20.

You tested the coolant sensor at 210 & 70 but not colder then that which is where it is struggling to start. I would test it first thing on a cold morning. The coolant sensor is a common failure part. But so is the IAC.

It sounds to me like the problem is caused by one or more parts that are marginal in operation. The ECM on these trucks will not set sensor codes unless the reading is outside the range on the top or bottom end. A coolant sensor could read 85 degrees all the time and this system would not set a trouble code.

You should test the fuel pressure with it running. The TBI trucks have idle problems when the pressure is low. The TBI trucks have no fuel pressure test port.

Also see if the scan tool you are using will show the P/N (park – neutral) switch information. The ECM uses P/N information to evaluate where the RPM should be, which affects IAC operation.
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Old 02-26-2011, 09:58 AM   #6
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Re: 87 Chevy 350 TBI low Idle issue

I had the same problem you described many years ago with my 87 350 FI 4x4 and after two mechanics played with everything with no results, I took it over and left it at the GM tech school and they found that the distributor shaft had become demagnitized. They replaced the distributor shaft and that fixed it and never had a problem again with it. It had the exact same symptoms you were describing and especially when shifting into reverse, it would just flat die. It would then start right back up but it was an occasional hickup and not all the time. I even had one GM dealer mechanic try and set the idle up by drilling out the plug to readjust the base plate. What a joke. Mark
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Old 02-26-2011, 10:16 AM   #7
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Re: 87 Chevy 350 TBI low Idle issue

I agree it could because of losing magnetism in the distributor.

Here is a reply I made on this topic in the past:

On these distributors the housing/upper bushing area can get so worn out that the shaft will move sideways so far that the points on the pickup coil pole piece get hit by the reluctor points on the shaft. When this happens the ECM receives compromised signal for RPM and timing issues.

Around 1989-1990 the distributor shafts didn't have the groove in the shaft to direct oil "back" down into the engine. Oil will get into the pickup coil and can damage the coil windings. Any of the distributors can do this if they get very worn.

If there is oil coming up the shaft and getting out onto the pickup coil – replace the distributor.

These distributors can also have problems because of loss of magnetism needed for the pickup coil to work.
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Last edited by ChevyTech; 02-26-2011 at 01:08 PM. Reason: poor wording
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Old 02-26-2011, 12:24 PM   #8
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Re: 87 Chevy 350 TBI low Idle issue

Another thing to check......how old is the fuel filter? Check the hoses for kinks. The low pressure of these means nything is a restriction.

How old is the coil? You can pick up a nice MSD coil which helps these engines for about $30.

Skip White has low cost distributors for TBI engines, look for his ebay store. Mixed reviews on them...some say they are good and some are not but the pric is low. Used ones are cheap at the junkyards too. Like Chevy Tech said, get a 91 - 95 unit.

At 171k miles I would also expect the TBI unit itself to be getting worn. They were not built for longevity. I am referring to the bottom half with the throttle plates. The shaft that the plate is on wears and gets sloppy. A good rebuild on this is between $150 and $200, good bushings on the shaft and all the passages will be cleaned out. You can also pick up used ones with less mileage if you look around.

Good luck and please post what solved the issue.
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Old 03-04-2011, 10:14 AM   #9
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Re: 87 Chevy 350 TBI low Idle issue

I figured I owed an update here. Currently I am on a budget so I can't just go and through parts at my truck so I have to do things when I can here.

First when I said I cleaned the area around the IAC, It was the area in and around the TBI and not the IAC. If I was going to clean the IAC I would only use a non corrosive, non residue electronic component or tuner cleaner, it being a stepper motor.

Currently I do not have access to a scanner though I have a friend who can borrow one when he has a free day from his job. So until then I can not check IAC steps or the P/N switch.

CTS resistance reads where it is supposed to be IAW with temp versus resistance. from 30 to 210 degrees. I have checked this a couple of times now and it is pretty consistent.

I am trying to borrow from the same place as the scan tool, a fuel pressure test rig. I have two tanks and the truck would do the same thing on either tank(or fuel pump). I went ahead and changed the fuel filter since I had not done it in a few years and it is something you should change somewhat regularly.

One thing I did find was my TPS was set at .65 volts in idle position. One article I read for this particular truck and engine, the TPS should be .54 plus or minus .075 volts and over 4.0 at WOT and voltage reading need to be smooth and linear throughout operation. This means I was high about .035 volts or so at idle. I adjusted it to .55 in idle position and 4.2 at WOT position. In either case the voltage was smooth and linear.

I do not know how to test the distributor for magnetism loss with out replacing it but I did pop the cap off it and there is no play either up and down or side to side and no oil in it. I cleaned the cap and rotor contacts and put it back together(very minor build up). I checked the timing and it was about or even less a quarter of a degree advanced position and the light or mark stayed steady.

I pulled a few spark plugs to check them. Gaps were still perfect and the electrode were the perfect tan or light brown color. Every wire I tested was getting a lot of fire at the sparkplug. The motor runs smooth and strong throughout the rpm range.

I tried to test to see if my motors rpm at idle kicks up a little when I engaged the AC, but either I have a bad pressure switch on my Receiver/dryer or I am out of freon since my AC comp will not kick on.

I sprayed inj/carb cleaner around the injector where the shaft for the butterflys are and did not get any idle change while the engine, so they still seem pretty tight.

With all the little stuff I have done the problem still exists but is no where as bad. For small trips about town it did not die at all. The only time now it seems to, is if you are at a light for a real long time or if you have to turn you steering wheel almost to the stops and keep it there for a while. I could not even get it to die in reverse which it used to readily before.

Once I get access to a scan tool and fuel pressure tester I will have more info and I will post that then.

The thing with out a doubt that has helped the most was the slight adj of the TPS (95 percent improvement) and changing the fuel filter (5 percent improvement). But something is still just a touch off. I am closer now to having this fixed than a couple of shops I had taken it to in the past.

Anyway, I will keep you informed .....
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Old 03-04-2011, 11:16 AM   #10
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Re: 87 Chevy 350 TBI low Idle issue

Thank you for the very thorough analysis. You are giving everyone who reads this a lot of good information on how to troubleshoot.

I was going to suggest the fuel pressure test might indicate a worn out fuel pump since they were never that great anyway but since it happens on both tanks I doubt that is a problem.

It did make me think of one possible item that might effect fuel pressure at idle and that is fuel lines. As I recall the first year or 2 of tbi trucks had some sections of fuel line that were rubber from the tank to the filter? (I may be wrong on this) There is a chance this line has gotten a little soft and is not holding the pressure as it should.

There is also the pressure regulator. Do those springs ever wear out and lose a little strength? The springs are under $15 as I recall. Maybe just cleaning and reinstalling will help the regulator.

I think the fuel pressure test will be very valuable.

Keep us informed.
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Old 03-05-2011, 08:38 AM   #11
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Re: 87 Chevy 350 TBI low Idle issue

The fuel pressure regulator is in the TBI unit.

You could have dual tanks with two perfect fuel pumps and still have low fuel pressure.

The fuel pressure regulator springs break.

Here is photo of a fuel pressure regulator spring from a vehicle with a surge:

Photo from this thread, which has some good information:
http://67-72chevytrucks.com/vboard/s...d.php?t=347247
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Old 09-11-2012, 08:17 AM   #12
Familytruxster
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Re: 87 Chevy 350 TBI low Idle issue

Birdman, was this issue ever resolved? My son's 91' GMC is doing everything you described.
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