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I recently bought a new (to me) 05 Impala that, as suggested in the title, was a police car. It had 155.6K miles on it, asking $3K cash. Since my Regal had blown its headgasket and had a myriad of problems wrong with it, I leaped on the chance.
Other than a few minor repairs I’ve been happy with the purchase. New front speakers, a cupholder, radio to listen to my weird music. However my first repair has popped up after 2-2 1/2 months of owning the car… the dreaded P0420 code.
Since a number of things can cause this, I’m asking some experienced guys with GM cars, namely the ones with 3800s, what are some things to look at. I do have some concerns and over the following week will try to get a better look at the problem.
Turkey Day I will be able to hook up a scan tool and have a chance to look at the O2 sensor data and have a rough idea what to look for, and plan on inducing a lean condition to see what happens. But I have some concerns.
The rubber in the radiator strut mounts allows for a fair amount of play in this car, so much that I actually feel it in the car as it accelerates or brake, or even turn. I’m concerned I have an exhaust leak due to the moving engine (and therefore, exhaust pipes) but havn’t had a chance to go climbing underneath the car. I also have no idea on how to tell if actually DO have an exhaust leak somewhere. Other concerns is if the cat is just barely below the overly sensitive threshold commonly reported on this car, how can I tell via sensor data? Another concern, the downstream O2 sensor around the nut portion is BLACK. Yeah, not kidding. Having sold auto parts for 4 years at Advance, I’ve seen my fair share of old Sensors, this does not look normal. I’ll have to get closer to make sure its not a trick of the light, but if it isn’t doesnt look good for the sensor. I also of course plan to hit it with my palm and check for broken pieces in the cat, but seems unlikely as it accelerates just fine and gas milage is pretty decent for city driving, I get about 170-200 miles on 8 gallons of gas.
Basicly I want to cover all the bases before actually trying to fix the problem. Thats just my nature. Look at all angles and then act.
PS: How can I tell if I have the HD or normal 4T65 transmission?
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a exhaust leak can cause bad 02 sensor readings, if its BEFORE the 02 sensor, so find out where your exhaust leak is, I will get you a video that Eric has on checking catalytic converter efficiency it covers well for your vehicle.
if you have the supercharged L-67 3800 engine or the 5.3L V8 and your car is newer I say than 1996 you have the 4t65eHD Transmission, if you have the N/A L-36 3800 you have the regular 4T65e, the major differences between the two transmission are some beefier internals in the HD, mainly the Limited slip differential is also not only limited slip but a little beefier in the HD, some of the splines for each gear are I think also hardened for the HD, the police interceptors in our town that are a impala typically have the V8… but I think those models are newer than 2005 so you may just have the supercharged engine, although I think even the N/A 3800 would make a decent interceptor engine.
for your catalytic converter effiency
for exhaust leaks
common know places for exhaust leaks for your car are the resonator, exhaust manifold gaskets sometimes, muffler, mostly leaks below the car are most noted as these are most exposed to the elements.
I’ve seen the video, but its on a 20 year old subaru, not a 9 year old impala, its easy to see that one is bad with the fast switching downstream O2 sensor. On the exhaust leak possibility, I suspect if I were to have one, its going to be where the manifold connects to the tail pipe assembly and constant rocking motion has eaten into the metal gasket. I hope to fix that on Turkey Day and turn the rubber portions up so the engine stops moving like that. Very annoying.
RE: the transmission, The Wiki article wasnt clear on what all the police package came with, but one would think they would beef up the transmission on a police interceptor, its a N/A 3800, so it stands a fair chance that its the regular transmission. Its just an extra I wouldn’t mind having making it more difficult to kill the transmission.
a good way to beef up that trans is to throw in the HD Diff… ZZP sells a kit to do that, comes with the diff, different CV shaft and other important parts, doubt they would put the HD transmission on the N/A 3800 but they could have installed it aftermarket, if you mean the rubber bushings for the dogbone struts that hold the engine steady leaving you feel they have too much play, the ones bolted to the front part of the body to the engine… naturally they have a lot of play for some reason, if you don’t like that there is a trick called “Flipping the mounts.” or “Flipping the dogbones.” you could once again go through ZZPs website and get the Polly bushings which are nice and solid.
Yes, flipping the mounts is what I plan to do. Like said, there is play, and who knows what it can do to an flange gasket, also its annoying while driving.
this is a good place to go if you want a little bit of aftermarket stuff, engine parts, transmission, turbo kits, suspension, brakes, this website pretty much shows you how to make your 3800 W Body a 11-13 second quarter mile car.
[quote=”Sovek” post=119812]Yes, flipping the mounts is what I plan to do. Like said, there is play, and who knows what it can do to an flange gasket, also its annoying while driving.[/quote]
if you suspect a gasket is causing the leak replace the gasket and make sure the mating sufaces for the two exhaust components are clean and smooth enough to make a seal, as far as the exhaust cross over that joins the front bank of cylinders with the back bank of cylinders, if I remember correctly I think there is no gaskets its sort of a tapered fit. when doing exhaust work use plenty of penetration oil such as Kroil or WD 40, liquid wrench or whatever you like… and flipping the bushings I think feels much more comfortable, I haven’t heard of it doing any damage to anything yet.
13aceofspades13 has forgotten more about a 3800 than I will ever know… most of the time a PO420 on a GM vehicle when I worked on them was a convertor problem…but since the post convertor O2 is black, I think too much unburned fuel is being dump into the exhaust.
Also…
RPO code MN7 Heavy duty transmission…RPO code M15 Enhanced electronics (transmission)i haven’t forgotten anything 😉 I suggested testing the catalytic converter with Erics catalytic converter test video, same procedure as any other vehicle if your 02 sensor readings are off as well as the temperature readings for the cat the cat is probably bad as Eric demonstrated, never assume always test ive even found this out the hard way on one of my cars. even as pattern as a 3800 W Body cars patterns can be, its always good to test first, the common issue is yes usually the catalytic converter or the 02 sensor and replacing either or will usually remedy the issue. but it can be a exhaust leak, wiring issues, a host of other problems, this is why I said if there is a exhaust leak find out where it is first.
however a Exhaust leak would point to a lean condition on the o2 sensor usually, most catalyst inefficiently codes are thrown because more hydrocarbons get past the catalytic converter because the downstream 02 sensor reads richer.
if you want to cheat ZZP sells little sensors for a couple of bucks you can plug into the 02 sensor slot that gives the PCM a fake reading and shuts the thing up 😉 but if where you live requires frequent inspections and emissions testing I would go with a brand new cat and see what happens honestly, but still sort out the exhaust leak first.
13aceofspades13 stated: never assume always test
That should be the quote of the day…. Thank you for saying that
13aceofspades13 stated: i haven’t forgotten anything
I should rephrase what I said; I wish I knew as much as 13aceofspades13 knows about a 3800 GM engine.
I hope I am not coming across as a know it all, that’s not my intention, sorry if I did just saying.
Pffft, I’ve had a 3800 for 7 years, I could not destroy that engine, the head gasket was probably a work in progress for at least a year if not more. Not to mention they arn’t bad to work on, up until you want to change out the plugs >.< Also, when I say the O2 sensor is black, I mean on the outside, not in the actual exhaust. I havn't pulled that off yet as I'm unsure what the exact issue is. But I was also under the impression that a bad cat, the output voltage on the O2 sensor will bottom out quite frequently causing issues. Course the front could be slow switching and causing the code. Ace, another question. Is it true that GM cars don't like Non-GM parts. I know with plugs on that engine that if you don't use OE plugs (I.E. Plat/Iridium) that you will wind up fouling them frequently with copper plugs due to that crazy emissions control setup they have. But there is talk of non AC-Delco parts (namely O2 sensors) not holding up very long. I know the front was replaced at some point, not sure on the back.
when it comes to spark plugs I recommend AC Delco, GM Cars like those best as the manufacturers tuned the ignition system around it, however NGK V Powers work well too, and I hear all the time how people have good luck with autolite… but I have heard not so good about Bosch with GM Cars, but have yet to see it for myself, but have had good luck with AC Delco and NGK. if you are looking for a good 02 sensor replacement can’t go wrong with the AC Delco unit.
as far as beyond spark plugs and 02 sensors like some electric components such as plug wires, ect, BWD Borg and Warner is awesome! when the evap purge solenoid went out on my car that’s what I replaced it with is a BWD unit, haven’t had bad luck with them yet. and MSD 8.5MM super conductor spark plug wires are the bomb, the 3800 loves them, however you have to sort of build them yourself and they are pricey.
when it comes to ignition coils stay away from Accel, experienced this myself, went through several ignition coils and went back too the stock AC Delco. but I have heard and have seen many modified 3800s run MSD ignition coils, people often have good to say about them.
Yeah, Having worked in a parts store I heard similar stories of the Bosch plugs being absolute junk in GM engines. Same goes for Autolite copper plugs, Think I had someone change the plugs with copper on one of those GM engines, came back in after the plugs fouled demanding a warranty refund.
Speaking of Evap canister, Fuel pump shuts off when unless I baby it when refueling. Best way to go about fixing this? No EVAP codes, just that minor annoyance. I can see the main line running from the gas tank, but not the one leading to the solenoid. weird.
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