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Dr.Automotive
Dr.Automotive, Auto Mechanic
Category: Chevy
Satisfied Customers: 3803
Experience:  Automotive Degree Dealer Experienced
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Suburban: the engine light comes on..accelerate..bogs down..backfire

Customer Question

Have a Suburban 1999 when the engine light comes on will not accelerate like it should. When accelerator is pressed it bogs down. if too much gas is given starts to backfire. The code reading is PO420. Had the catalitic ck is ok the fuel pump is ok, the oxigen sensors are relativelly new and ck ok as well. The technitian sayd the problem was with the ignition failling to advance.In the past I have had tis problem and when the engine light goes out (on its own)all goes back to normal. It would work for two or three weeks then light comes on, and back to the same no power. The lack of power is most noticiable when engine is cool like in the morning. the more gas is given the less it goes. once gas is released it picks up some speed. At speed the vehicle stumbles if gas is given sudenlly. Do I need to purchase a new ignition?. This is what was recomended by the technitian who ckecked the suburban.
Submitted: 3 years ago.
Category: Chevy
Expert:  Gary replied 3 years ago.
so the code PO420 is still current and setting?
Customer: replied 3 years ago.
Yes, code still there I tryed to clear to no avail,
Expert:  Gary replied 3 years ago.
ok the 420 code is showing there is a definite problem with the cat converter.NOW that may be the problem right there or not.it would depend if it was broken up inside or the catalyst has failed inside it.either way that MUST be corrected first before any other diagnosis can be done due to that converter problem will effect ANY tests that are done to the truck.like i said that may fix it right there or NOT.but any test you do without fixing that cat converter.i wish i had a quick fix for you but that cat has to be fixed before anything else
Customer: replied 3 years ago.
Well the technitian at Minique said that he did a ck on both cat converters and they are ok. If the problem is with the cat then how come after running the car for a few days all gets clear?
Expert:  Gary replied 3 years ago.
it all depends on how the cat is bad.there is no way those cats check out if you have that code.they might not be physically broken up where you hear them rattle but if they are not working right that code will set.that code is saying the computer monitors the O2 content coming into and out of the cat.with that code its saying the same reading is being read before AND after the cat.which means either its melted inside/broken or the cats efficiency is no good.which will effect fuel control
Customer: replied 3 years ago.
What, can I do to get the gas problem fix if the code is still there i will replace the cats .Hate to blow awy $450.then realize that that by replacing the ignition control unit would have corrected the problem.
Expert:  Gary replied 3 years ago.
like i said it may fix the problem or not
Customer: replied 3 years ago.

It sounds to me like it is falling to advance the ignition, that is what the technitian stated, I have not connected the timing light on it . If the cat is bad will it prevent the ignition to advance,

Also when the light goes out then all runs like new. then a couple of weeks some times month lights comes on , And all goes bad. If the cats are going bad or if bad the the engine lite would stay on all the time would not clear on its own right??

Expert:  Dr.Automotive replied 3 years ago.
I know the problem is intermittent but this is 100 PERCENT a catalytic converter in the process of completely failing. You need to get this fixed as soon as possible before it leaves you stranded. Do the steps below thanks and have a great evening.

Danny

1. Check for engine misfire.

2. Check the fuel pressure regulator for a leak.

3. Check for coolant consumption.

4. Replace the catalytic converters.
Customer: replied 3 years ago.

Well no misfire beatifull idle---

fuel press at 65-----

coolant will have to ck.

what if there is a coolant loss??

guess ill replace the cats and hope for the best.

Expert:  Dr.Automotive replied 3 years ago.
You can see if the cat is bad if coolant has high exhaust reading by getting test strips and testing the coolant. I am 100% Positive the cats are the problem. Let me know if you need anything else. Please hit the accept button so I can get awarded for my time and effort. You can follow up on this question for free with me. I know its frustrating when its seems impossible to be the problem but I am telling you this is the problem and these cars are very sensitive so if there is anything else I can do let me know..


Danny
Dr.Automotive, Auto Mechanic
Category: Chevy
Satisfied Customers: 3803
Experience: Automotive Degree Dealer Experienced
Dr.Automotive and 2 other Chevy Specialists are ready to help you
Customer: replied 3 years ago.
Ok. One more question. does the cat or the ox sensors detect readings that are not acceptable, will this prevent the timing not to work or there might be another problem in the picture in addition to the cdat going bad?
Expert:  Dr.Automotive replied 3 years ago.
bad cat will cause all of the reading to be off O2 sensor MAF and the PCM will try to do what it can to fix the issues but can only do so much. Read this should tell you what you need to know about your CAT system and why you need a new one is cause its faulty.

graphic
Customer: replied 3 years ago.

OK. Catalitic repraced. Engine light went out. GOOD sign, right??

well I still have the same problem, still bogs out when gas pedal is depressed.

I believe ignition is not advancing as is needed. Is the ign control module inside the ignition assembly or is an exterior module. The ign cap is good the rotor is good. She cranks up really good and the idle is right at 500 rpm. Can't get the timing light on it the damper to see if is advancing or not. The timing chain was repleced about 8 month ago. Now whAt???

Expert:  Dr.Automotive replied 3 years ago.
So you cleared the codes and they are now gone we have eliminated one of the 2 problems good. Let see if we can check the timing. If everything is done and still having problems then we will need to inspect the crank sensor for failure.
graphic

TEST PROCEDURE
The ignition timing cannot be adjusted. The distributor may need adjusting to prevent crossfire. To insure proper alignment of the distributor, perform the following:
  1. With the ignition OFF, connect the scan tool.
  2. Start the engine. Allow the engine to idle until the engine reaches normal operating temperature.

IMPORTANT: Cam Retard Offset reading will not be accurate below 1,000 RPM .

  1. Increase engine speed to 1,000 RPM while performing the following steps.
  2. Use the scan tool in order to monitor the Cam Retard Offset.
  3. If the Cam Retard indicates a value of 0 degrees , the distributor is properly adjusted.
  4. If the Cam Retard does not indicate 0 degrees , the distributor must be adjusted.

ADJUSTING PROCEDURE

  1. With the engine OFF, slightly loosen the distributor hold down bolt.

IMPORTANT: Cam Retard Offset reading will not be accurate below 1,000 RPM .

  1. Start the engine and raise the engine speed to 1,000 RPM .
  2. Use the scan tool in order to monitor the Cam Retard Offset.
  3. Rotate the distributor as follows:
    • To compensate for a negative reading, rotate the distributor in the counterclockwise direction.
    • To compensate for a positive reading, rotate the distributor in the clockwise direction.
  1. Repeat step 4 until 0 degrees is obtained.
  2. Turn OFF the ignition.

NOTE: Refer to Fastener Notice in Cautions and Notices.

  1. Tighten the distributor hold-down bolt.

Tighten
Tighten the bolt to 3 N.m (25 lb ft) .

  1. Start the engine.
  2. Raise the engine speed to 1,000 RPM and view the Camshaft Retard Offset.
Customer: replied 3 years ago.
I'll try that. will let you know tomorrow, thanks, Jose
Expert:  Dr.Automotive replied 3 years ago.
Ok
Customer: replied 3 years ago.

I Ok. I am using an ACTRON AUTO SCANNER. Can't find camshaft retard offset.

I did get a 0.0 degrees ign advance with engine off, and 23. degrees advance with the engine at 1000 RPM. if press accelerator quick drop to 7 to 8 degrees, however if accelerator is pressed slowlly it will go to 28 degrees advance. Still i do not get any diagnostic trouble. I have a list of reading that I got with engine off and with it running. i can fax to you or send apicture of these. Thanks Jose

Expert:  Dr.Automotive replied 3 years ago.
Yeah I would like to see this upload to photobucket and send me a link


Danny
Customer: replied 3 years ago.
graphicI do not know if this works. If you have an E-Mail it would go really fast-at least for me
Expert:  Dr.Automotive replied 3 years ago.
Site policy no email.. Just go to www.photobucket.com then upload and send me a link
Customer: replied 3 years ago.
Photobucket wont let me sign in. Wont let me join in. Is there another way. this is a very long list. I could try to send in here.So bear with me.
Expert:  Dr.Automotive replied 3 years ago.
Give it a shot!!
Customer: replied 3 years ago.

OK. ongine off. Mil status off,- ABSLT TPS 0.0- eng speed 0.0- calc load 0.8- map (lb/m) 0.23-MAP (HG) 28.9-coolant 165- IAT 108- ign adv 0.0 fuel syst 1 open- o2s11 0.115- o2s12 0.80- o2s21 0.140- o2s22 0.085---

Engine running.. IAT 105- ign aDV 23-23.5- O2S11 0.22 -O2S12 0.050 -O2S21 0.265 -O2S22 0.120 (OX SENSORS FLUCTUATE A LITLE BETWEEN .050 TO 0.743== ENGINE SPEED650= CALC LOAD 3.1 = mAP (LB/m) 0.98 COOLANT 192 iat 105 VEHICLE SPEED 0.0. wHAT ELSE SHOULD I CK.

Customer: replied 3 years ago.
wHY I CAN PRESS ACCELERATOR SLOWLLY AND IGN ADV GOES UP ALTHE WAY TO 28 (MAXIMIMUN ADVANCE RIGHT). bUT IF iS PRESSED QUICKLY IGN ADV GOES TO 5 6 DEGREES WONT RESPONT UNTIL PEDAL IS RELEACED. ???
Expert:  Dr.Automotive replied 3 years ago.
The problem is the Vehicle Control Module on this vehicle Thats what actually controls the timing advance. This can be reprogrammed by dealer software but dont know if thats gonna fix the problem. Here is the location and I know this is it there is nothing else that controls the advance its all done by this bad boy on this truck!

graphic
Customer: replied 3 years ago.

OK. I ghess I will have to go to the dealer. I was told that had to leave the car overnight and they will perform a thorough test That it would take 4 maibe 6 hr for the to determine the problem @ $98 an hr. then the parts that would be neede to repair, they would not guarantee tha this would correct the problem since is an old vehicle....... that to me is a lot OF B-- S---. Thanks for your help.. Jose

Expert:  Dr.Automotive replied 3 years ago.
I honestly would get a used VCM and throw that in worse case you have to get it programmed which is less than 1 hour of labor. This has to be your problem dont do that diagnostics we already have done that. The VCM is controlling the advance and is either out of calibration or needs to be replaced. Get one from a salvage yard or car-parts.com
Customer: replied 3 years ago.

What is a V C M. can't find in the book.

Thanks Jose

Expert:  Dr.Automotive replied 3 years ago.
The Vehicle Control Module
Customer: replied 3 years ago.
Sorry I'm so frustrated that I'm not thinking. The VCM vehicl controll unit that must cost a fortune. I'll se if the dealer will accept the reprograming of it. or if I can find one in a junk jard.
Expert:  Dr.Automotive replied 3 years ago.
This is not much at the salvage yards and www.car-parts.com is a online database of salvage yards they will reprogram it.
Customer: replied 3 years ago.

Danny, Jose , went to the dealer to have them ck the VCM, well it ck OK. the mechanic stated there was a bigger problem, after 1.5 hrs of trouble checking found out that the FUEL FILTER was the problem, Correct pressure however wrong VOLUME of fuel, It maques sence now, controll module saw not enoght fuel when the fuel was demanded by the engine, it was retarding the ignition to compensate for a lean fuel mix. YOu can learn something new everyday. I never though of volume and pressure, both have to be there. They ran a complete diagnostic on the engine. the technitian admited that this was the only last thing that could came to His mind, fuel quantity.

Was charged for 1 hr labor and $18.00 for filter.

Thanks for all your help. Jose

Expert:  Dr.Automotive replied 3 years ago.
Sweet deal. Glad everything is good!!


Danny
Customer: replied 3 years ago.

Qh I forgot was charged $1.00 disposal and taxes $7.00 still cheap.

At least I know now I have a good vehicle for anothe 100,000 miles. I hope.

Thanks Jose

Expert:  Dr.Automotive replied 3 years ago.
Yep Let me know in the future if you need anything else. Thanks

Danny

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