1994 k1500 wild misfire

[name]

Well I got to summit racing, got some parts, the condition is still there :

1) Replaced cap and rotor with the ones that came with the new aftermarket distributor I mentioned earlier = No effect on misfire at idle .

2) Replaced wires with summit brand witch I had good luck with in the past = No effect on misfire at idle .

3) Replaced coil with a summit brand that was listed as a 30,000 volt coil ( witch I figure is probably what the OEM's are ) = No effect on misfire at idle .

4) Cleaned up the old plugs and re gaped them at .035 just to eliminate problems with the new CR43TS and re set timing to 4* BTDC = No effect on misfire at idle .

5) Decided to recheck my work and re clean MAP sensor and EGR solinoid with electrical cleaner, when I took apart the plug from the map sensor I broke them round plastic deviders that surround the terminals, soliderd in a new plug = No effect on misfire at idle .

6) Grabbed a can of WD-40 and just started spraying in every vaccume hose I could find and even down the TBI unit itself, the RPM's kicked up a little and it appeared to smooth out a little, but I did disconnect the battery and I am not sure if it was the result of the WD-40 or the ECM learning where it likes to be .


My questions are :

How long does it take the ECM to settle down and figure what it wants to do after disconnecting the battery ?, It seems like it will work the idle down to 500 rpm in park and about 250 rpm in gear when it is warmed up ( is this to low ? ) There is no drivability issues up to 70 MPH and it seems to like the 4* advance around town , but every time I come to a stop it is fine for about 1-2 minuits and it starts to have a random miss, ( not like it is missing on the same cylinder on every rotation ) the longer I sit at idle the worse and more frequent it gets. once I take off and drive it for a while then stop again it is good for the first 1-2 minuits then starts to act up again.

It is not throwing any SES lite or loging any codes, I did put a timing lite on each plug wire to check for iregular flashes t kinda looks like one of the wires flashes more than the others but I am not 100% sure, I did re rout this wire but doing so did not have any effect on the miss.

Just before I did this tune up I cleaned up the known grounds under rthe dash, frame and inside engine bay area, when I did that I also added a couple of ground wires to firewall to fender and frame to fender, then I started reading another post where they are talking about a " ground loop " What is a ground loop and could this be my problem ?

 

[Schurkey]

5) Decided to recheck my work and re clean MAP sensor and EGR solinoid with electrical cleaner, when I took apart the plug from the map sensor I broke them round plastic deviders that surround the terminals, soliderd in a new plug = No effect on misfire at idle .

MAP or MAF? I have no idea how you'd clean a MAP sensor.

6) Grabbed a can of WD-40 and just started spraying in every vaccume hose I could find and even down the TBI unit itself,

WD-40 is not the right product for that. Aerosol carb cleaner or throttle-body cleaner, would be more reasonable.

How long does it take the ECM to settle down and figure what it wants to do after disconnecting the battery ?

Up to 50 miles of varied driving--in town, highway, etc., is what I've heard. Potentially longer than that if it involves the computer doing OBD2 testing of the various systems. But you don't have OBD2, so the relearn should go relatively fast and easy.

The scan tool will tell you if the fueling is "learned" or not.

It seems like it will work the idle down to 500 rpm in park and about 250 rpm in gear when it is warmed up ( is this to low ? )

WAY too low. Have you ever dicked with the minimum idle speed screw, hidden behind the tamper-resistant cover (at least until someone removes the tamper-resistant cover.)

Does this have the idle vacuum solenoid and vacuum throttle kicker? Does it work?

every time I come to a stop it is fine for about 1-2 minuits and it starts to have a random miss, ( not like it is missing on the same cylinder on every rotation )

How do you know it's not the same cylinder?

the longer I sit at idle the worse and more frequent it gets. once I take off and drive it for a while then stop again it is good for the first 1-2 minuits then starts to act up again.

Connect a scan tool, find out what the sensors are showing after a minute or two of idling.

I started reading another post where they are talking about a " ground loop " What is a ground loop and could this be my problem ?

A ground loop is very unlikely to cause a misfire only after a minute or two of idling.

 

[name]

MAP or MAF? I have no idea how you'd clean a MAP sensor.

MAP Remove MAP sensor, hose down with electrical contact cleaner including the port , let dry and reinstall

WD-40 is not the right product for that. Aerosol carb cleaner or throttle-body cleaner, would be more reasonable.

I used carb cleaner in the beginning but it had no effect, then I read a post saying it was not good to use carb cleaner on the sensors, decided to re do the work with electrical cleaner witch is suppose to be the same as MAF cleaner, used wd-40 to check for vacume leaks because I heard it works for that.

WAY too low. Have you ever dicked with the minimum idle speed screw, hidden behind the tamper-resistant cover (at least until someone removes the tamper-resistant cover.)

Does this have the idle vacuum solenoid and vacuum throttle kicker? Does it work?

This unit did not idle this low before I did the tune up, when I found the broken magnet in the dizzy and and needed to replace it I decided to pull all the stuff that might make me more room to get it in correctly. I am disabled and crawling up on that engine and shimmying my fat *** all the way back there was not something I was looking forward to. I pulled the EGR, MAP sensor and bracket, the coil and the IAC. I replaced the EGR because it was throwing a code, cleaned the MAP sensor as described above and also cleaned the IAC. But when I cleaned the IAC with carb cleaner I started screwing the ram in and out, Not knowing how they really worked or If I messed it up I simply threw in a new one.

After everything was put back together the unit would idle way high and throw an IAC code. After reaserching this new problem I found that the pigtail will fit into the IAC 90* out if the clip is missing and cause a way to high idle condition. After re intsalling the plug the correct way the idle would come down to the above mentiond RPM's witch were a lot closer to where the unit orginally idled at, so I called it good and moved on looking for the soure of my original problem witch was the miss at idle .

I have never messed with the idle speed screw, the cover plate for the adjustment screw is still in place so I figure it has never been touched by a PO either . I have no clue if this unit has a vaccume solenoid or throttle kicker, But there is something mounted in the front of the intake by the thermostate housing that has an electrical connection and a hose that goes to the evap canister. No clue what it is or does though...

How do you know it's not the same cylinder?

If it was the same cylinder, on every rotation, then I should see that on the timming light.

 

[Schurkey]

This unit did not idle this low before I did the tune up... ...I pulled the EGR, MAP sensor and bracket, the coil and the IAC...

...I found that the pigtail will fit into the IAC 90* out if the clip is missing and cause a way to high idle condition. After re intsalling the plug the correct way the idle would come down to the above mentiond RPM's

I'd be looking in the service manual for the proper procedure for replacing the IAC.

One wonders if it's now messed-up due to the connector being installed improperly.

I have never messed with the idle speed screw, the cover plate for the adjustment screw is still in place so I figure it has never been touched by a PO either .

Excellent!

If it was the same cylinder, on every rotation, then I should see that on the timming light.

Not if it's a compression or fuel problem. You might if it was an ignition issue.

 

[name]

For those of you following this thread :

1) I re set the IAC as described in the service manual = no effect on misfire at idle

2) I installed a new bosh o2 sensor = no effect on misfire at idle

3) I replaced the TBI gasket = Limited effect on misfire at idle.


I decided to pull the TBI and take a look at the gasket in an attempt to find my misfire and low idle issue, notes from the PO says it was replaced when they did the intake gasket, here is what I found but I do not know what would be " normal " there was 80k miles on this TBI gasket when I pulled it .

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The channel that goes for the PCV valve was pretty crusted up and its counter part on the gasket looked to be compromised. I removed the gasket only to find a thin layer of Black RTV gasket maker between it and the intake. I cleaned it up by soaking it in gas and scraping it with my thumb nail, ten block sanding it with 240 grit sandpaper.

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This went pretty easy but I only sanded enough to clean up the mounting surface of the gasket maker, it appears that there were machining marks left on the intake from the factory, and some mild pitting in the surface area itself. I can only guess that this is why the PO used gasket maker but what baffels me is the mounting surface on the TBI itself is way worse with casting defects, but no gasket maker was used between the TBI and gasket.

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This is what they looked like when I put them together with a new gasket and without any gasket maker. Another thing to note is all the star bolts on the injector pods were not very tight, I don't think I would even call them "snug" and one of the TPS bolts were not even tight enough to butt up against the plastic housing. I snugged them pretty good before putting it all back together.


1) Is the rtv gasket maker normal between the intake and TBI gasket and should I have used it Again ?

2) After installing and tighting everything down I still have a low idle, can I adjust for this by tweaking the position of the TPS ?
( the mounting holes seem slotted from the factory )

 

[docstoy]

First question on last post, OE never used rtv on throttle body or gasket, just clean flat surfaces, if you have areas that are not flat then could be from improper removal of gasket or debris, you can fill areas that aren't flat but remove excess so you will have flat surfaces.
Second question on last post, If you oem throttle body it didn't have adjustable tps. Should be able to find desired tps voltage spec. in gm service manual for driveability diagnosis .

 

[Schurkey]

1) I re set the IAC as described in the service manual = no effect on misfire at idle

Does the IAC work after having been plugged-in incorrectly?

I removed the gasket only to find a thin layer of Black RTV gasket maker between it and the intake. I cleaned it up by soaking it in gas and scraping it with my thumb nail...

1) Is the rtv gasket maker normal between the intake and TBI gasket and should I have used it Again ?

NO! RTV gasket maker has NO BUSINESS being applied to a gasket. It's a gasket MAKER, not a gasket SEALER. It is used INSTEAD of a gasket, not in addition to a gasket.

You've also discovered that RTV tends to deteriorate in the presence of gasoline. So it has no business anywhere in the presence of gasoline.

 

[1998_K1500_Sub]

I cleaned it up by soaking it in gas and scraping it with my thumb nail, ten block sanding it with 240 grit sandpaper.

This went pretty easy but I only sanded enough to clean up the mounting surface...

What steps did you take to remove debris from the intake passages after sanding the mounting surface?

Another thing to note is all the star bolts on the injector pods were not very tight, I don't think I would even call them "snug" and one of the TPS bolts were not even tight enough to butt up against the plastic housing. I snugged them pretty good before putting it all back together.

There are torque specs for these parts, and likely they're in the factory service manual.

I would apply the torque specs, here, and on any bolts which join parts involving a gasket seal.

2) After installing and tighting everything down I still have a low idle, can I adjust for this by tweaking the position of the TPS ?
( the mounting holes seem slotted from the factory )

Idle speed is controlled by airflow, at least when everything else is working as it should. Tweaking the TPS is not a solution.

If your IAC (idle air control) valve is disconnected or malfunctioning, it may be the culprit.

I assume the default setting of the throttle blades is unchanged from "factory", because you said earlier the idle speed screw has not been altered, so I would expect them to "bleed" the factory-spec'd amount of air. Thus, idle RPM would be completely under the control of the ECU via the IAC valve, if the ECU & IAC are working correctly.

I ... also cleaned the IAC. But when I cleaned the IAC with carb cleaner I started screwing the ram in and out, Not knowing how they really worked or If I messed it up I simply threw in a new one.

Confirm that the pintle on the new IAC valve has the same form and dimensions as that on the former IAC valve. The pintle has to have a specific shape to work as designed with the corresponding bleed hole in the body.

The picture below is for reference. The pintle on your IAC(s) may or may not look exactly like the picture.

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I also added a couple of ground wires to firewall to fender and frame to fender, then I started reading another post where they are talking about a " ground loop " What is a ground loop and could this be my problem ?

If you want to rule-out a grounding problem, then arrange your grounding wires / straps exactly as GM did. Don't go adding additional ground conductors in other locations willy-nilly, they aren't generally necessary and may introduce an issue. Ask a question(s) if you want/need to know more.

You can potentially improve the factory grounds by replacing the conductors (if corroded or if known to be inadequate) and cleaning the connections / connectors and attaching screws / bolts (if rusted, corroded).

The factory has AFAIK one ground wire from battery "-" to the fender, which exists on the passenger's side.

There are two ground straps near the passenger's side firewall, one from the frame to a boss the firewall and another from the same boss on the firewall to the passenger's side head on the engine.

There's a ground wire of heavy gauge from battery "-" to the engine.

Sometimes there's a ground strap from the firewall to the hood.

Ground straps are used instead of wires because they have lower series inductance. They're used instead of wire for a reason, so don't try to out-smart yourself or GM when repairing / replacing.


I'll give you points for being a thinker and a doer, and asking questions. It's obvious you don't know everything, but who does?

 

[Schurkey]

Ground straps are used instead of wires because they have lower series inductance. They're used instead of wire for a reason, so don't try to out-smart yourself or GM when repairing / replacing.

Thanks for that. Something I'd never considered.

 

[name]

Does the IAC work after having been plugged-in incorrectly?

I can only assume so , It does change idle speed from being cold and being warmed up