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Old 07-14-2009, 10:12 AM   #1
Free88
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1990 Chevy 4x4 low speed miss

Truck starts good but idles a little rough for about a minute than smooths out. Runs rough until about 40 mph. Over 40 it runs well with no roughness at all. Had the throttle body gasket changed and it made no difference. Does not idle fast.
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Old 07-14-2009, 12:09 PM   #2
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Re: 1990 Chevy 4x4 low speed miss

Somebody will pick up on this and make some suggestions, but there are a boatload of posts from within the past month that deal with rough idle, etc., and may offer you some good tips for troubleshooting. I'll check out some links and throw one your way.
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Old 07-14-2009, 12:11 PM   #3
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Re: 1990 Chevy 4x4 low speed miss

Check this post. Go down towards the bottom and there is a reply from ChevyTech that has some info that may help.

http://67-72chevytrucks.com/vboard/s...d.php?t=348308
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Old 07-14-2009, 02:03 PM   #4
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Re: 1990 Chevy 4x4 low speed miss

Thanks.
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Old 07-14-2009, 07:26 PM   #5
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Re: 1990 Chevy 4x4 low speed miss

It sounds to me like weak ignition or a distributor issue.

On these distributors the housing/upper bushing area gets 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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Old 07-14-2009, 07:27 PM   #6
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Re: 1990 Chevy 4x4 low speed miss

Does anyone have a part number for this spring. We don't have a Chevy dealer where I live and I have to call a dealer and don't really want to stay on long distance hold while they look up the part. Thanks
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Old 07-14-2009, 07:30 PM   #7
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Re: 1990 Chevy 4x4 low speed miss

Quote:
Originally Posted by Free88 View Post
Does anyone have a part number for this spring. We don't have a Chevy dealer where I live and I have to call a dealer and don't really want to stay on long distance hold while they look up the part. Thanks
Are you referring to a fuel pressure regulator spring?
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Old 07-14-2009, 07:38 PM   #8
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Re: 1990 Chevy 4x4 low speed miss

Yes sir that is exactly what I am talking about. Sorry I was not more specific.
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Old 07-14-2009, 07:41 PM   #9
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Re: 1990 Chevy 4x4 low speed miss

Last time I check the fuel pressure regulator spring could not be bought separately at the dealer, and the only way they sold it was with the cover assembly, and it was about $150 if I remember correctly.

I get them at the U-Pull-It salvage yards.

Some people are buying them from internet sites.
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Old 07-14-2009, 07:55 PM   #10
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Re: 1990 Chevy 4x4 low speed miss

Thank you Chevytech. Your help is much appreciated. I will try to find one.
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Old 07-14-2009, 08:05 PM   #11
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Re: 1990 Chevy 4x4 low speed miss

You’re welcome.

Let us know how things go. I will try to keep an eye on this thread.

Note: I have been busy and cannot make it to this board every day right now.
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Old 07-23-2009, 04:18 PM   #12
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Re: 1990 Chevy 4x4 low speed miss

Well here I go again. Changed throttle body to one I know that worked. Changed fuel filter, and also put in a new base gasket. Truck still bogs and bucks until it hits about 40 and then it smoothen's out. Checked for vacuum leaks at throttle body and all over engine, and found none. Changed wires, cap rotor and plugs also. Distributor has no slop in it.
Could the timing chain have jumped a tooth causing this low speed roughness? I am puzzled, any other ideas?


Thanks
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Old 07-23-2009, 08:59 PM   #13
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Re: 1990 Chevy 4x4 low speed miss

Quote:
Could the timing chain have jumped a tooth causing this low speed roughness? I am puzzled, any other ideas?
These engines have steel timing gears with no nylon teeth. I have never seen a chain jump on one of these.

Being we are not sure what direction to look, here are a few things you should check:

Check the ECM for trouble codes. Let me know if you need instructions on checking for codes without a scan tool.

You could try disconnecting and plugging the hose to the EGR valve and taking it for a test drive.

Testing the coolant temp sensor is easy. Do a search with ChevyTech and coolant sensor and you should find specs and instructions.

If you have found no problem the next thing to check is fuel pressure and can post information on this if you want it.
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Old 07-23-2009, 09:19 PM   #14
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Re: 1990 Chevy 4x4 low speed miss

ChevyTech. I am not getting any lights showing up on the cluster. I will take it tomorrow and have a friend of mine plug in his code checker and see if the ecm has stored any codes. I will disconnect the EGR in the morning and take it for a run and see if it smooths out. I appreciate all your help.
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Old 07-23-2009, 10:02 PM   #15
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Re: 1990 Chevy 4x4 low speed miss

If it won’t flash out a code the ECM could be bad and running in a backup (limp home) mode. That would explain it running poorly.
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Old 07-23-2009, 10:12 PM   #16
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Re: 1990 Chevy 4x4 low speed miss

What I meant was that there is no check engine light on. I did not realize that it would store codes without some kind of warning light on the dash. I will look for the post on how to test for codes without a code reader. Sorry if I confused things.
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Old 07-24-2009, 08:29 AM   #17
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Re: 1990 Chevy 4x4 low speed miss

Here are some web sites with the basics on retrieving codes without a scan tool or code reader.

Poor drawing of ALDL but ok instruction + codes
http://www.cfm-tech.com/gm_tbi_trouble_codes.htm

OBDI terminal identification
http://robertpowersmotorsports.frees.../ALDL_plug.jpg

OBDI illustration and instructions
http://www.troublecodes.net/GM/

1985 – 1990 trouble codes instructions
http://www.iroczone.com/specs/TroubleCodes.htm
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Old 07-24-2009, 09:16 AM   #18
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Re: 1990 Chevy 4x4 low speed miss

Thanks. I will try this as soon as I get a chance today. Right now I am going to block off the egr valve and take it for a run. If that does not work I am going to swap ecm's ( computer? ) and try that. Are they different from a 305 to a 350?
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Old 07-24-2009, 09:36 AM   #19
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Re: 1990 Chevy 4x4 low speed miss

Look at the ECM numbers.
The same ECMs where used for both engines for a number of years but the chips (Prom) would be different, but look at the numbers on the ECM. Depending on the vehicle and equipment some vehicles used a different ECM.
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Old 07-24-2009, 06:11 PM   #20
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Re: 1990 Chevy 4x4 low speed miss

Thanks for all the help. I think we are closing in on the problem. Due to all the work on the 91 transmission today I was not able to do much with my truck. Wifes 91 always comes first. Happy wife, good life. So I disconnected the egr valve today and this eliminated a lot of the problem. Still a little rough but not as bad. I also noticed a hole in the y pipe today just close to the oxygen sensor. Wondering if that may have anything to do with it. Anyways I am going to the city tomorrow and will be back to the truck either Sunday or Monday. I will let you know how it is after I remove the EGR and clean the valve and port. Have a good weekend and enjoy your time with family. Thanks.
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Old 07-24-2009, 08:32 PM   #21
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Re: 1990 Chevy 4x4 low speed miss

You're welcome.
Quote:
So I disconnected the egr valve today and this eliminated a lot of the problem.
Note: A lean condition caused by another problem can be magnified when the EGR valve opens.

EGR valves have a spring in them that can get weak, or break, causing the valve to open too much or to easily.

I would suggest only using a general motors replacement EGR valve, if you decide to put a new valve on it.

If the fuel pressure is low, this will exaggerate the sag in power when the EGR opens.


Quote:
I also noticed a hole in the y pipe today just close to the oxygen sensor. Wondering if that may have anything to do with it.
Yes!
Fix the leak.
Air getting in the exhaust system can also affect the o2 reading. In open loop operation (cold) the ECM ignores the o2 sensor reading, but in closed loop (warm) the ECM uses the o2 reading to determine if the engine is running lean or rich. If the exhaust system leaks ahead of the o2 sensor, (or just behind it), the pulses in the exhaust will draw in air and cause the o2 sensor to get an incorrect reading. A vehicle with an A.I.R. will have the same problem if the air management valve allows air to be injected in the exhaust manifolds during closed loop operation.
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Old 08-01-2009, 07:36 PM   #22
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Re: 1990 Chevy 4x4 low speed miss

ChevyTech, today while crawling around under my truck to see where the noise was coming from I noticed that the sensor in the exhaust pipe had no wires coming from it. I checked all over for a while but could not find one. I looked in the manual and it showed a purple wire in the schematic number 412 going to connector D7 on the ecm. I can not locate this wire and was wondering if you had any idea as to where this may be located. It looks like a two wire connector on the O2 sensor but only shows one wire in the schematic. Thanks
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Old 08-02-2009, 09:30 AM   #23
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Re: 1990 Chevy 4x4 low speed miss

It should be a one wire sensor on your 1990 truck and the wire should be Purple.

On that truck the O2 sensor should have a single wire to a connector the in several inches from the actual sensor.

I can’t remember which years have the sensor in which positions. It may be that the exhaust system on your truck is from something else. Look at the driver side exhaust manifold and make sure there is not oxygen sensor there also, close to the outlet.

The single wire O2 sensors have to be very close to the engine to stay warm enough to function well.

Last edited by ChevyTech; 08-02-2009 at 09:45 AM. Reason: Add more
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Old 08-02-2009, 09:59 AM   #24
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Re: 1990 Chevy 4x4 low speed miss

Good morning. There is a sensor on the drivers side exhaust manifold and this one was in the exhaust manifold. I am going out to check and see what sensor is in the exhaust manifold on the drivers side. The sensor I took out has 4 wires on it.
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Old 08-02-2009, 10:18 AM   #25
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Re: 1990 Chevy 4x4 low speed miss

That is a four wire heated sensor which is NOT for your truck.
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