Light Throttle Surge

[k5hart]

Hello Guys,

I recently purchased a 1991 Silverado with 5.7 and automatic trans. The truck runs like a sowing machine except for one annoying problem. At very light throttle, and I mean just barely, the truck will surge. It won't shut off but comes very close and if you give it just a little more throttle it stops. I've changed the complete distributor and coil, throttle posistion sensor, coolent temp sensor, and fixed the wires going to the coolent sensor where the shielding had come off. It has new plugs, good wires, and I just put some fuel injection cleaner in this week. I pulled the battery when I changed the sensors to allow the PCM to reset. Checked the timing is set at 0.

It seemed to go away after changing the coolent temp sensor for the first drive down the street and then it came back. All other driving situations it runs great. Does kind of idle high a lot in park to me, but it might just be me.

Any ideas? Thanks for the help!

[ChevyTech]

Here are a few thoughts:

Replace the fuel filter if you have not done that yet.

I would look at the base gasket under the TBI unit for a vacuum leak.

Next I would test the fuel pressure.

You could try disconnecting and plugging the hose to the EGR valve and taking it for a test drive.
Note: A lean condition caused by another problem can be magnified when the EGR valve opens.

[k5hart]

Thanks for the advice. I'll give those things a try when I can.

Would the TBI gasket causing a vacuum leak, in turn also causing the higher idle?

What's the best method to check the fuel pressure? I don't remember seeing a test port.

[ChevyTech]

Would the TBI gasket causing a vacuum leak, in turn also causing the higher idle?

Yes
On this system the idle speed is controlled by how much air the IAC (Idle Air Control) lets in, and a vacuum leak, if bigger then a very small leak, will cause the computer to lose control of the idle speed.
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.

What's the best method to check the fuel pressure? I don't remember seeing a test port.

Here is some info about fuel pressure testing I saved from a previous reply I made to a similar post. Hope the links still work.

C/K truck TBI systems do not have a fuel pressure test port, but the pressure must be tested with the engine running.

The fuel pressure is tested by using fittings to “T” in a pressure gauge. I like to T in the pressure gauge where the fuel filter is located. If the pressure is good, both the pump and regulator are ok. If the pressure is low it could be the pump or the regulator. If the test is done with a method that allows the fuel filter to be in place, a dirty filter can also cause low pressure. The regulator is inside the TBI unit. Stopping the flow in the return line momentarily will test to see if a low pressure problem is caused by the regulator. A bad regulator can let the fuel return to the tank instead of maintaining the correct pressure. If pinching off the return line, using care not to damage the return line hose, makes a low pressure reading surge above the specification pressure, then the regulator is the problem.

The TBI fuel pressure specification for small block Chevy’s is 9 – 13 PSI with the truck running.

Here are some web sites showing fuel pressure test equipment. The first tool gets installed where the fuel filter goes. Most people use the tool in the third site next to the TBI unit.

I you already have a pressure gauge like shown in the second web site below, all you need is the tool shown on the first web site. If not, the third would get the pressure tested for the least money.

http://www.sjdiscounttools.com/ta37650.html

http://www.actron.com/product_detail.php?pid=16174

http://www.actron.com/product_detail.php?pid=16175

http://www.cfm-tech.com/catalog/fuel...er_2940807.htm

Another addition to the list:
http://www.tradervar.com/cgi-bin/sto...;page=3641.htm

[k5hart]

Thanks. Very informative. I have my work cut out for me.

[k5hart]

Alright, I'm bringing up an old thread here. I kind of put the this work on the back burner since it wasn't top priority. This week I ordered a fuel gauge adapter from cfm-tech and put it on today.

First I have to say that this is a cool product and a great company. I put on a new gasket while I had the throttle body off. So I have found that I have 8 psi fuel pressure. 1 psi under the lower side spec. I changed the fuel filter with no change. I pinched off the fuel return line and fuel pressure jumped up to 14 psi.

So can I take this as a bad fuel pressure regulator? Anything else that I'm not thinking about? If it's the regulator has anyone used the adjustable ones that are available. Is there any true benefit to using an adjustable regulator on a bone stock truck?

[bwood]

Originally Posted by k5hart

If it's the regulator has anyone used the adjustable ones that are available. Is there any true benefit to using an adjustable regulator on a bone stock truck?

I have read where many people have run the adjustable regulator kits. All of the applications I have read about were performance applications where a higher fuel pressure was needed. If you are all stock, I would just use normal replacement parts.

[ChevyTech]

If pinching the return line made the pressure go to 14 PSI it does indicted the regulator is not doing its job.

The pressure should have jumped up higher with the return pinched if it has a good strong pump.

I see no point in putting and adjustable regulator on the stock system. If the pressure is low (like 8PSI) there is a problem and sooner or later it will give you more trouble, so you may as well find the problem and fix it, rather then modifying things to work around it.

I would look at the spring in the regulator and see if it is broken, but my guess is the pump in your truck is weak and will give you more trouble.

Sorry about my delay in getting back to your post. I have been traveling and have not had time to get to the message board.

[k5hart]

My gauge doesn't go any higher than 14 psi so it may be more. It was pegged with the line pinched. I did buy the adjustable regulator because I have plans on rebuilding the engine eventually with some modifications. I figured why not just do it the first time. Got it last night and what I didn't realize was it isn't really a replacement for the factory regulator, just a modification.

So I guess I will still need a replacement regulator? So, you think there is also a problem with the fuel pump? I really didn't want to have to change that. I doesn't make any noise and I don't have any other drivability trouble.

[ChevyTech]

If you gauge only goes to 14 PSI, your pump may be ok.

The most common failure with the regulator is the spring breaks.

GM does not sell the spring separately. Gm sells the top of the TBI with regulator, spring, and diaphragm all assembled.

I get regulator springs from the local “you-pull-it”.

I have seen regulator springs sold online but they do not look like the factory springs.


Note: the service manuals state that the fuel regulator should not be disassembled for safety reasons.
Realize there is a spring inside so if you take the screws out without holding it together, it could hit you in the face.

I would look in the regulator and see if the spring is broken.

[k5hart]

Well I drove around 60 mph for about 30 minutes constant two ways last night and truck started idling really bad. I looked at the pressure guage and it was at 8 psi. Guess something the problem is getting worse and is going to force me to fix it. It always happens at the bad times! New spring in the mail, adjustable mod is already here. May rebuild the throttle body while it's out.

[ChevLoRay]

Ain't it funny the way we go about trouble-shooting our mechanical issues? For example, I drive a school bus with air brakes. I've had a problem with it showing sufficient air pressure yet getting an alarm that the air pressure is low....upon cold start-up. Talked to the mechanic. Was told that the compressor is going bad....doh. Was also told that when it quit they would replace it. Okay, I'm hauling a load of kids on a bus. If the compressor quits, the air pressure drops and when it drops the brakes set because there is no air pressure to over-ride the springs on the spring brakes. Now, where would a good place be for THAT to happen...? Luckily, before leaving on my morning route, the air pressure wouldn't build up enough to close the air-operated door. Guages showed good pressure, but working the treadle made the air go away....bad air....bad air.

Sounds like your trouble-shooting is like mine most of the time....seat of the pants. Hope it all works to your advantage.

[k5hart]

Yikes! Wait for the brakes to fail before fixing them!! I just don't understand peoples thinking. Glad the compressor failed while stopped. I used to be more proactive and on the ball about problems with my cars but with a new baby time is at a minimum. Go figure, drove it to work today and its running great.

[ChevLoRay]

Got it back this afternoon. Gooooood air pressure. I like my air brakes, a different sort than the air brakes Earnest had on Santas' sleigh in "Earnest saves Christmas"...

[k5hart]

Well, I rebuilt the throttle body today. Replaced the fuel pressure spring which was in 4 pieces. Added an adjustable regulator mod. I have 11 PSI fuel pressure.

I still have the surging at light throttle! What else can it be?? I'm running out of stuff to replace. I have to say I'm a little disappointed.

[ChevyTech]

I the regulator spring was broken in 4 pieces, you did not waste your time working on it, even if the surge is not gone.

Originally Posted by ChevyTech

You could try disconnecting and plugging the hose to the EGR valve and taking it for a test drive.
Note: A lean condition caused by another problem can be magnified when the EGR valve opens.

Did you try a test drive with no EGR after you did the TBI repair?

Is it a very slight surge or a bad surge?

Do you ever hear the engine ping (detonate)?

Have you ever poured Sea Foam or an equivalent down the TBI to clean carbon from the valves?

Have you ever had the intake off this engine and looked at the valves to see how bad the deposits on them are?

[k5hart]

I wondered if the EGR was the next thing to try. I'll try it disconnected.

It's a very bad surge but only at very very light throttle. Like the amount you would use to back out of the driveway or creep in traffic. It also shows up slighty around 30 mph, cruising, with the trans in 4th and convertor locked up. No load on the motor. A little more throttle or letting all the way off clears it right up.

I've got to be close. Not much more to change! Could it be the IAC?

No I haven't been into the engine or used Seafoam. According to the previous owner the heads were remove at some time and freshened up. Who knows what actually got done. It really runs great except for this annoying problem. I wouldn't worry about it so much if it didn't jerk at 30 mph. A lot of my driving is in town and it gets annoying.

Everything I fix seems to make it a little better. The throttle body actually gave me my power back at higher rpms and it is smoother feeling. Your right it wasn't a waste. I can't believe this was even working.

[k5hart]

By the way Chevytech. Thanks for all the help! It's nice to have someone knowledgable to run stuff by.

[ChevyTech]

By the way Chevytech. Thanks for all the help! It's nice to have someone knowledgable to run stuff by.

You’re welcome.
Sorry for slow replies – I have been so busy I can only answer a few posts.

Nice photo: The EGR springs also fail, and then the EGR opens too easily, and to far.

The IAC should not cause this problem, and you already replaced the TPS, so my best guess at this point is EGR.

I would suggest only using a general motors OEM (Original equipment manufacturer) replacement EGR valve, if you decide to put a new valve on it. The TBI trucks are very picky about their EGR operation.

[k5hart]

Ok. So I disconnected the vacuum line for the EGR at the throttle body and drove down the road a little bit. The problem is gone. Now could it be the EGR solenoid? Or probably the valve? I'm going to double check the lines too. Man I hope this is it!

[ChevyTech]

There are multiple types of EGR valves.

TBI trucks have either Negative pressure EGR or Port EGR systems, except California emission 4.3 TBI trucks had Linear EGR valves.

Negative pressure EGR valves are the most common.

Negative pressure EGR valves - Should have an “N” after the last numbers on the valve. The pressure in the exhaust system effects (modulates) how much the valve opens.

Port EGR Valves – The EGR solenoid pulses like a mixture control solenoid/fuel injector to control vacuum supply. Port valves have no “P” or “N” and it is blank after the last numbers on the valve.

Here is a site with a good illustration of a negative backpressure EGR valve.
http://shbox.com/1/EGR_valve.jpg

This is a site that has a photo and an illustration about EGR vale numbers which should help you determine which type of valve your vehicle has.

http://shbox.com/1/EGR_ID.jpg

Negative backpressure valves should hold vacuum with the engine off. When the engine is started the valve should bleed vacuum. The slightest pulse of pressure in the exhaust system will modulate recirculation (close the valve at least some). A good test for these is to put a hand vacuum pump on the EGR valve and apply vacuum with the engine off and it should hold vacuum. Leaving the vacuum on it, start the engine, and the valve should close.

Now could it be the EGR solenoid?

If your truck has a negative back pressure EGR valve, the solenoid does not modulate the opening of the valve and it is clearly working if disconnecting the EGR made a difference. The vent portion of the solenoid can get plugged but that is not near as common as the EGR valve getting bad.

On negative backpressure EGR systems the ECM powers the EGR solenoid when the engine might need EGR operation and it is the internal mechanism in the Negative backpressure EGR valve that modulates how much the EGR valve opens.

If you find your truck has a Port EGR valve let me know and I will give you information on the Port system valves.

It is very common for negative back pressure EGR valves to fail on these TBI trucks.

Once again - I would suggest only using a general motors OEM (Original equipment manufacturer) replacement EGR valve, if you put a new valve on it.

[BIGphillip79]

I have the same problem and it seems by, the number of threads on the same topic, that its pretty common. I haven't checked the spring yet but I've been on a quest to find one locally. So far no luck.
As far as EGR, I've got an "89 K1500 with a 5.7. Its a new motor and the EGR was put on new and has maybe 5000 miles on it. I got the truck not running so don't know if it was surging before. I was thinking of removing the EGR all together like I've done on all my previous engines. Of course they were all carbed. What would the efeect be on the tbi engine?

[k5hart]

I believe I have a port EGR system. I found instructions on testing them in my Haynes manual. I hooked up a vacuum pump to the EGR valve with the truck running and applied vacuum. The truck stumbled and almost shut off. This indicates proper operation according to the manual. Checked the EGR solenoid by applying the vacuum pump to it. Manual says it should maintain vacuum and it wouldn't hold at all. So I think I'm looking at the solenoid as being bad. Am I on the right track?

BIGPhil - I think if you remove the EGR you can experience some spark knock unless the computer is recalibrated to not look for the EGR. That being said, I've had mine unhooked since the weekend and haven't had any problems. But, I'm only driving in town a couple miles a day so I'm probably not a good judge. Rebuilding the throttle body was the best thing I think I could have done. It was bound to fail soon and very easy to do. It was in bad shape. I got my spring off Ebay from Mr.Injection.

[BIGphillip79]

Yeah, I saw the Mr Injector store and there was another, TBI Performance. My uncle said he had some TBI units laying around and I could get what I needed, but its probably no better than what I got and wouldn't last. I'll probably just order the spring though.

[ChevyTech]

Checked the EGR solenoid by applying the vacuum pump to it. Manual says it should maintain vacuum and it wouldn't hold at all. So I think I'm looking at the solenoid as being bad. Am I on the right track?

It depends which side of it you put vacuum on.

With the engine off the EGR solenoid should not pass any vacuum if a hand vacuum pump is put on the TBI side of the solenoid.

How many wires are the going to the EGR solenoid?