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Old 04-24-2021, 06:57 AM   #1
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1991 30' Airstream 30
Cape Elizabeth , Maine
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1990 454 TBI Very Rough Idle

Engine runs extremely rough at idle but is fine at speed, power is normal. We just completed a cross country trip of almost 7000 miles. Ran great all trip until the last 2 days. Over those 2 days the idle progressed from a slight issue into the condition it is in now. Felt like either a bad spark plug or wire. Exhaust looks OK and the fuel consumption is normal.

Here is what has been done between myself on the last day of the trip and the shop that is now working on it:

Replaced ICU, MAP, Distributor cap, Rotor, spark plugs and wires.
Fuel from tank was drained and new fuel added.

It did throw a code for the EGRV and that is the next thing the shop will work on. The mechanic agreed that a bad EGRV can cause a rough idle but not as bad as it is now.

After reading some other posts on this forum it looks like timing could also be an issue, I did not realize the timing chain can jump a tooth so I will have them check the timing.

I'll provide an update once the EGRV and timing are looked into. Any other areas I should into?

One other thing, the TAC will sporadically jump 500-1000 RPM when on the highway, usually from 2900/3000 to 3500-4000while at constant speed. Not sure if this is related to idle problem or not. TAC appears to indicate correct RPM at less than highway speeds.
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Old 04-24-2021, 07:29 AM   #2
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How many miles on the engine. It could be time to change the timing chain due to normal wear if it's a high mileage engine. The job isn't too hard if it's in a pickup.
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Old 04-24-2021, 07:37 AM   #3
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How many miles on the coach?
Temp sensor at front of intake manifold, could be going bad. Does the exhaust smell rich?
Pickup coil in the distributor could be going bad!?
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Old 04-24-2021, 07:41 AM   #4
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Quote:
Originally Posted by weatherbie View Post
Engine runs extremely rough at idle but is fine at speed, power is normal. We just completed a cross country trip of almost 7000 miles. Ran great all trip until the last 2 days. Over those 2 days the idle progressed from a slight issue into the condition it is in now. Felt like either a bad spark plug or wire. Exhaust looks OK and the fuel consumption is normal.

Here is what has been done between myself on the last day of the trip and the shop that is now working on it:

Replaced ICU, MAP, Distributor cap, Rotor, spark plugs and wires.
Fuel from tank was drained and new fuel added.

It did throw a code for the EGRV and that is the next thing the shop will work on. The mechanic agreed that a bad EGRV can cause a rough idle but not as bad as it is now.

After reading some other posts on this forum it looks like timing could also be an issue, I did not realize the timing chain can jump a tooth so I will have them check the timing.

I'll provide an update once the EGRV and timing are looked into. Any other areas I should into?

One other thing, the TAC will sporadically jump 500-1000 RPM when on the highway, usually from 2900/3000 to 3500-4000while at constant speed. Not sure if this is related to idle problem or not. TAC appears to indicate correct RPM at less than highway speeds.

I would look at the Ignition distributor pickup inside of the distributor. I learned that it can cause a variety of problems, incl hard starts and idle issues. I fortunately replaced the entire distributor, because upon closer inspection, I found that my distributor had a broken internal shaft that would have gone undetected if I would not have taken it apart.

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Old 04-24-2021, 09:47 AM   #5
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1991 30' Airstream 30
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It has 95000 miles, forgot to add that in the original post.

The temp sensor is relatively new. I remember when that went bad and the rough idle it has now is not like it was when the sensor went bad. Exhaust is good.

I'll mention the distributor when I talk to the mechanic monday.

Thanks for the ideas.
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Old 04-24-2021, 10:22 AM   #6
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Smooth It Out!

At this point I would install a whole new distributor! This is a well known fail point in GM TBI engines. A new parts store OEM is less than $100 I believe but I would go for an aftermarket high performance model! This is one of the first things to get replaced when someone is trying to get their TBI to run better!
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Old 04-24-2021, 10:38 AM   #7
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Definitely reaching back to years ago, but would a dash mounted vacuum gage be a handy diagnostic tool on these engines?

Sorry, spark plug free here since 2005.
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Old 04-24-2021, 11:15 AM   #8
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Years ago we had a chrysler 454 town and country station wagon that lost power while on the hiway. the 3rd or 4th dealership had a mechanic that pulled the fuel pump and found that between the fuel pump and the cam that operated the pump was a small shaft. It was worn and not to spec. a friend of his was rebuilding a 454 and sold us a new shaft, problem solved. you aren't loosing power. just my 2 cents worth.
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Old 04-24-2021, 11:54 AM   #9
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1991 30' Airstream 30
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Quote:
Originally Posted by mykytiukr View Post
At this point I would install a whole new distributor! This is a well known fail point in GM TBI engines. A new parts store OEM is less than $100 I believe but I would go for an aftermarket high performance model! This is one of the first things to get replaced when someone is trying to get their TBI to run better!
I think that's where I am heading, thanks.
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Old 04-25-2021, 02:16 PM   #10
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I think that's where I am heading, thanks.
Times two, on Peter's suggestion. I would replace that and most of the stuff hanging off the TBI e.g. rebuild kit, EGR valve etc which is cheap and a decent insurance policy. Keep the old working spares.

Have a look back at some of our posts which will shed some light on it and rough running on the TBI. One other thing you might consider is bluetooth ALDL reader which will show you the underlying data in your ECM. This might also yield useful info. Supplier is
John Wales
1320 Electronics LLC
I have no relationship with the fella.

Keep us posted and hit us up for any more intell.

Good luck

Nick
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Old 04-29-2021, 02:51 PM   #11
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1991 30' Airstream 30
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An update - On Monday I told them to check the timing and if that was ok then replace the distributor. It has been slow going, the foreman at the truck shop is the only one who is familiar with distributor ignitions and he finally got to it this afternoon. They called this late this afternoon and the distributor has play in it so tomorrow it will be replaced. I'm hoping this does the trick.
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Old 04-29-2021, 03:41 PM   #12
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454 and 95k miles on original timing chain

Your past due for a timing chain/gear replacement. While your at it put on a new water pump as it has to come off to get at chain. You will be shocked to see how sloppy that chain is when you pull the front cover. Take a dry and wet compression check. That will show if intake/exhaust valves are seating properly. Do a leak down test on each cylinder to judge the piston ring condition. 95k miles on a 454 in a coach like yours with a low axle ratio is probably getting a bit on the tired side. Maybe a crate 454 from a reputable (think Jasper Engines) is in your future.

As a drag racer from back in the day I ran the P___ out of a bunch of 454's. Good engines but they have there weaknesses.
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Old 05-04-2021, 11:52 AM   #13
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A new distributor was installed and the timing is good, no change in the rough idle. Compression check is next.
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Old 05-12-2021, 12:38 PM   #14
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1991 30' Airstream 30
Cape Elizabeth , Maine
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Compression check went fine and the mechanic finally decided to check the EGRV, which I told them at the beginning appeared to be the problem. Lo and behold the valve was stuck open. With the new one installed it now runs fine.

I appreciate the suggestions and have learned much from them. As soon as I return home from a current unplanned trip I intend to put it up for sale. If anyone knows someone who might be looking for one please put them in touch with me on the forum.

Cheers, Max
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Old 07-24-2021, 12:12 PM   #15
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1991 30' Airstream 30
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I guess I spoke too soon. The idle is still a little off, it searches a little and seems just a little rough. The engine is now showing a code 43 which I understand is the ESC and indicates the knock sensor detected a knock.

According to my manual the sensor should be on the passenger side of the engine and the ESC near the TBI. I have been unable to locate either, any suggestions?

If the timing is good can the timing chain still be an issue?
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Old 07-24-2021, 12:32 PM   #16
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Quote:
Originally Posted by weatherbie View Post
I guess I spoke too soon. The idle is still a little off, it searches a little and seems just a little rough. The engine is now showing a code 43 which I understand is the ESC and indicates the knock sensor detected a knock.

According to my manual the sensor should be on the passenger side of the engine and the ESC near the TBI. I have been unable to locate either, any suggestions?

If the timing is good can the timing chain still be an issue?
Valve timing and ignition timing are two separate things, the timing chain turns the cam and opens/closes the valves at the right time while the distributor timing creates the spark signal at the right time. It is very easy to check timing chain slack without disassembling anything, you just need a socket to fit the crank bolt, a degree wheel (or just use the timing marks on the bracket) and a helper to watch the rotor.

Has anyone checked for vacuum or intake leaks with propane or a smoke machine?
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Old 07-24-2021, 02:40 PM   #17
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Austin , Texas
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Quote:
Originally Posted by weatherbie View Post
I guess I spoke too soon. The idle is still a little off, it searches a little and seems just a little rough. The engine is now showing a code 43 which I understand is the ESC and indicates the knock sensor detected a knock.

According to my manual the sensor should be on the passenger side of the engine and the ESC near the TBI. I have been unable to locate either, any suggestions?

If the timing is good can the timing chain still be an issue?

On my 454, I think the knock sensor with a single wire is on the drivers above the manifold towards the rear.
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Old 07-25-2021, 06:32 AM   #18
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1991 30' Airstream 30
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Valve timing and ignition timing are two separate things, the timing chain turns the cam and opens/closes the valves at the right time while the distributor timing creates the spark signal at the right time. It is very easy to check timing chain slack without disassembling anything, you just need a socket to fit the crank bolt, a degree wheel (or just use the timing marks on the bracket) and a helper to watch the rotor.

Has anyone checked for vacuum or intake leaks with propane or a smoke machine?
I have thought about vacuum leaks but testing for those is beyond my ability. Tomorrow I start looking for a good mechanic who has worked on these older engines.
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Old 07-25-2021, 06:33 AM   #19
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1991 30' Airstream 30
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On my 454, I think the knock sensor with a single wire is on the drivers above the manifold towards the rear.
Thanks, I'll look in that area today.
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Old 07-25-2021, 04:27 PM   #20
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Quote:
Originally Posted by weatherbie View Post
....

Replaced ICU, MAP, Distributor cap, Rotor, spark plugs and wires.
F...

Is the ICU for Idle air control valve, or IAC?

I went through 3 Ignition Modules, AcDelco made in China
is this it:
https://smile.amazon.com/ACDelco-217...s%2C272&sr=8-4
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