rough surging when cold only( Bobbinski ?)

[bobnsue2]

Hi,

I've asked this before and no help, so I'm hoping bboinski might jump in.

91 STS. I replaced the injectors 6 months ago and the fpr with used low mileage injectors. It ran great since then. But recently it started acting up real bad when first started. After it warms up it runs great.

When it is first started it surges and bucks, feels like it will stall then the throttle blips up and it runs faster. It does this on and off for several minutes. Some mornings longer than others.

The injector flow and balance tests run fine.

Also setting no codes.

I swapped the map out becasuse from what I figure in the manual, when it is cold and still open loop, the map and coolant temp are what control the mixture. It seemed to be worse after that, so I did a relearn. Almost felt like it was fixed, but it is still doing it but not as bad and not every morning.

Any thoughts or ideas?

Thanks,

Bob B

[bobnsue2]

more:

It's a 4.9 and fuel pressure stays good while doing this.

Bob B

[Scotty]

The no codes and only cold confuses the issue.

I had an intemittant TPS on my 91 but it gave full time idle hunting

The ISC motor doesn't cause surging

It has to be something that doesn't set a code, I would:

You checked the fuel pressure when cold, so its not a balky fuel pump, FPR

How about fuel filter

Check the calibration of the coolant sensor (probably would set a code)

Check for an intake manifold or vacuum leak that seals itself when the engine warms up by spraying water at the junction I would put a vacuum guage on it

I had a bad MAP on a Bonneville that did this

Just a few thoughts, sorry I jumped in, you requested bbobynski, I was hesitant to repond.

[bobnsue2]

Thanks for jumping in.

I need any help and hoping for Guru.

The pressure is fine warm or cold. (new FPR)

One difference between cold and warmed up is open loon as opposed to closed loop operation. So I'm thinking open loop might be the area of the problem.

The reason I mentioned the cold/warm, is it only runs crappy when cold. After 5-10 minutes, it smooths out and runs great.

I did swap the MAP but no help.

This is my wife's car and she refuses to drive it until it is fixed.

Bob B

[Scotty]

There are a lot of components that can cause this. But you are not getting a code, so that makes if more difficult. Is the MAP new? I would put a vacuum guage on the engine and see what you get

[bobnsue2]

There are a lot of components that can cause this. But you are not getting a code, so that makes if more difficult. Is the MAP new? I would put a vacuum guage on the engine and see what you get

Ok,

Ran it with the vacuum gauge and here's the results:

running smooth = 18.

goose it and it drops to 0 then up to 23, then settles back on 17/18.

While it is chuggling, the vac drops and recovers when the isc gives it some gas.

In 7-8 minutes it warms up and runs smooth and goes into closed loop.

That running crappy during open loop must be telling me something.

Bob B

[Scotty]

I would check your O2 sensors I have been thinking that your O2 sensors are slow to react and they don't set a code as a result. I was doing some research on this and found this:

The slots in the tip of the O2 sensor can also get partially clogged with carbon which will increase the response time. This will cause the O2 voltage change to slow way down, taking 3-4 seconds to go up and down, instead of changing in less than a second. This slow response can cause a varying idle speed and varying idle mixture. The exhaust gas Carbon Monoxide (CO) reading will not be steady at idle when read by an exhaust analyzer. This can also cause some light surging under light acceleration with the engine cold and when fully warmed up under cruise conditions

[bobnsue2]

OK

Thanks I'll mess with them and let you know.

Bob B

[KevinW]

The ECM/PCM does not use oxygen sensor feedback to control the fuel mixture during open loop operation. While the surging is occurring, enter diagnostic mode and monitor the closed-throttle switch status (fan 'LO' illuminated indicates a closed switch as I recall).

[bobnsue2]

Scotty,

BTW, I wonder why BBobinski hasen't added any thoughts to this? Maybe 4.9 non north star doesn't get his attention any more.

Ok,I ran the diagnostics and payed close attention to the O2 sensors.

They switch fast and the cross count for both is 6 when faster than idle.

But,,,

The shop manual says that in open loop, the O2 sensors are not used.

Just the MAP and coolant temp. then after it warms up enought it goes into closed loop. That seems to be when it starts to run good.

I did swap out the map and watching the temp it looks to be going up as you would expect.

I wonder if the injectors could somehow be doing this, it's darned expensive to get others to find out. I checked the resistance of them and they are all real close to 16 ohms each.

The balance test is good as is the flow test run while cold.

This is a real clean no rust California car and I still am getting very tired of it.

My wife wil not drive it this way so she takes my GMC pickup. But Oh well, I drive my old 69 Land Rover (wish it would snow!) which I do enjoy a lot. Both the GMC and Land Rover use regular gas so that is a plus now. The STS uses premium.

Bob B

With a 4.9 sts heading for the back yard soon.

[adallak]

The STS uses premium.

I have been using 89 for two years, and there was no noticable change neither in performance nor in fuel economy. The latter was up to 27.4 during long highway round trips last summer. The economy dropped to 25 with new tires.

[Scotty]

Was the MAP new?

[Logan]

My old '93 use to have a mild bog for the first 400 feet or so after a cold start......very mild....after that it was fine.

Be sure to inspect your ignition module and AC pick up inside the distributer. Its a common issue to have 'green' terminals after this period of time.

'Green' terminal connections will often show up @ cold starts then be ok for the rest of the day issues.

[bobnsue2]

Well,

Logan's suggestion about corroded connectors in the distributor was good.

after removing the cap, I found the center electrode (carbon rod and spring) had broken and was turning around in there eating up the cap. So new cap and rotor with fingers crossed.

No joy though.

But it has prevented a future breakdown.

After doing another relearn, it runs great, but then it is nicely warmed up by then.

Then in the morning it stumbles and tries to stall until warm

I'm going to add a can of Techron injector cleaner in case they are causing this crappy running when cold.

If it ruins the injectors I don't much care at this point. I'm getting very tired of no diagnostic indicators pointing at anything.

Oh!, Scotty, the MAP is a used one but I got two at the pick and pull for a few dollars so I tried the other one. Made no difference.

Bob B

[maydog]

Have you checked the EGR valve and passages, a slightly stuck open valve may cause some issues until the vehicle is warm. Do you have any issues under a hard acceleration?

You also may want to check the temperature gauge to make sure it is accurate when cold.

[Logan]

What I wanted you to inspect was.....not the rotor....but rather the ignition module electrical connections and the AC pickup coil located at the base of the distrbutor.

See attached pic. Each end of the ignition module has connections that can get goofy over time. Also the AC pickup coil can get 'green.'

[Logan]

I should add....I know....the pic is a much older style GM HEI unit. But it does show the ignition module and AC coil.

[bobnsue2]

OK,

Here's the latest:

I found the center electrode in the distributor cap was broken. So I replaced the cap and rotor.

Two days later, the coil and module died so they are now new. Apparently I lost the groung strap that mounts on one of the coil mount screws and into the center slot of the cap connector. It ran bad still and eventually died totally. I can't find the strap anywhere though and think it might already have been missing.

I did go through the injector diagnostics again while cold in the AM. They check out fine.

This roughness while cold feels like a fuel delivery problem.

So I noticed the fuel pressure was stating at 40-42 lbs. no matter what I did with the throttle. So I took off the vacuum hose to it and lo and behild I find the sucker to be leaking. It's only a year and a half old.

Now the vac hose is disconnected and a golf tee plugged into the hose to the throttle body.

The pressure stays at 40-42. and it still idles like crap no change at all.

I also plugged the hose to the power brakes and replaced the PCV. still no help.

Obviously I need to order a Fuel pressure regulator.

But I think it should run good at 40 lbs. OR,, is that too much??

Scotty, Any thoughts?

Even with the vac hose removed from the Regulator, it runs good after warming up.

And goes like heck.

Nextm aaybe tomorrow, I will remove the throttle body and egr valve to do a visual under there.

Bob B

[Ranger]

Obviously I need to order a Fuel pressure regulator.

But I think it should run good at 40 lbs. OR,, is that too much??

Scotty, Any thoughts?

40 PSI is too little. It should have 45-50 PSI.

[Scotty]

OK,

Here's the latest:

I found the center electrode in the distributor cap was broken. So I replaced the cap and rotor.

Two days later, the coil and module died so they are now new. Apparently I lost the groung strap that mounts on one of the coil mount screws and into the center slot of the cap connector. It ran bad still and eventually died totally. I can't find the strap anywhere though and think it might already have been missing.

I did go through the injector diagnostics again while cold in the AM. They check out fine.

This roughness while cold feels like a fuel delivery problem.

So I noticed the fuel pressure was stating at 40-42 lbs. no matter what I did with the throttle. So I took off the vacuum hose to it and lo and behild I find the sucker to be leaking. It's only a year and a half old.

Now the vac hose is disconnected and a golf tee plugged into the hose to the throttle body.

The pressure stays at 40-42. and it still idles like crap no change at all.

I also plugged the hose to the power brakes and replaced the PCV. still no help.

Obviously I need to order a Fuel pressure regulator.

But I think it should run good at 40 lbs. OR,, is that too much??

Scotty, Any thoughts?

Even with the vac hose removed from the Regulator, it runs good after warming up.

And goes like heck.

Nextm aaybe tomorrow, I will remove the throttle body and egr valve to do a visual under there.

Bob B

If you have fuel at the vacuum port of the FPR, I would replace it and see what happens, it seems that you have found a few items that are contributing to your problem, maybe that is the reason why you are maintaining the same pressure. mike

[bobnsue2]

I have the throttle body and egr off. The egr is tight with no leaking. The throttle body was grungy under the blades where you can't see or hardly clean. But with it off, I really got it clean.

With the throttle body off, the egr was easy to remove. Didn't even need a special wrench.

Ordered gaskets from NAPA. Then on line discovered that Napa has a fuel pressure regulator from Delphi for 55.00. Outstanding better than gmpartsdirect by a lot. I understand Delphi is a spinoff of GM so it should be close or same as Delco.

I will be reassembling it in two days. Then we'll see what goes. Of course another relearn. This puppy ought to be getting real smart with all the relearning going on.

Ok, Ranger,

My shop manual indicated 38 to 50 lbs.

Don't forget this is a 4.9, not a north star.

Thanks for all the help.

Bob B

[Scotty]

I discussed this recently with Matt, its WAY easier to clean the TB when you take it off, i used my dremel with a brass brush to help clean it... Good Luck, next time you need parts try www.gmotors.com for a quote, I got my FPR for about $45. Mike

[Ranger]

Ok, Ranger,

My shop manual indicated 38 to 50 lbs.

Don't forget this is a 4.9, not a north star.

Thanks for all the help.

Bob B

It's been a few years since I had the 4.9 and I have since sold the manual so I was going from memory. I could have sworn it was the same. Sorry for the wrong info.